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Strongly related: How do we deal with octuplicate questions?

This is not a question about how to deal with duplicates, it's a simple "meta" question:

What is the most rampant duplicate on Stack Overflow?

  • The particular duplicate must be "alive" on Stack Overflow (not merged or closed duplicate)
  • The duplicate-ness must be pretty unquestionable.

For example:

This is a duplicate of score 2 1.

They are both so close that it makes little sense to have separate questions.

However, score 2 is pretty uninteresting.

(The rationale of this question is to find the duplicates so we can get the administrators to merge them. It is also to find better ways to find dupes in the first place, and perhaps turn finding duplicates into a game.)

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How is the "score" of a duplicate determined? The total number of duplicates? –  Kyle Cronin Jul 26 '09 at 5:51
Yerp, total number of "live" duplicates –  waffles Jul 26 '09 at 5:54
With true irony, is this a duplicate? meta.stackexchange.com/questions/9050/… –  Marc Gravell Jul 26 '09 at 8:55
Marc, this is about the search for the most rampant duplicate, which is one possible solution to the duplicate problem. its not about solving the problem, it a much narrower, and hopefully more fun question. –  waffles Jul 26 '09 at 11:35
One that comes up again and again is floating point comparisons e.g. "why is if (myFloat == 23) not working?" –  ChrisF May 17 '10 at 11:54
@mmyers is correct, this question is itself a dupe: meta.stackexchange.com/questions/26621/… –  The Unhandled Exception May 17 '10 at 22:19
@Josh: No, this question came months before the other. –  mmyers May 18 '10 at 1:01
@mmyers: Oh, you're right. Sorry! :-) –  The Unhandled Exception May 18 '10 at 3:23
See this question for another 5 candidates + 2 more in the answers. –  Jørn Schou-Rode May 24 '10 at 15:01
I'm pretty sure "Regular Expression Help" gets asked most often, but we always change its title to something a bit more descriptive. –  Bill the Lizard Jan 6 '11 at 0:45
Fantastic question -- at last a rep-incentive for closing dupes, and look ma no code changes! (Even if it's only meta-rep.) IMHO it should be a requirement to list all the question URLs in your answer to get upvotes here. –  j_random_hacker Jan 26 '11 at 8:06
Ironic that I also posted a duplicate: meta.stackexchange.com/questions/52530/… –  George Stocker Oct 5 '12 at 0:00

25 Answers 25

How to parse HTML with RegEx?

...by far has the most dupes.

... just to name a few.

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This answer is underrated! –  The Unhandled Exception May 17 '10 at 14:45
Exact duplicate of meta.stackexchange.com/questions/26621/… –  mmyers May 17 '10 at 22:12
@mmyers - too funny. I will upvote in 1 hour –  The Unhandled Exception May 17 '10 at 22:20
@George: Good answer, but in order to make this question all it can be (namely: a real life dupe-findin', rep-rewardin' machine) I'm -1ing until you give up the URLs. –  j_random_hacker Jan 26 '11 at 8:08
@j_random: There. I added some. –  uɐɯsO uɐɥʇɐN Jan 30 '11 at 20:29
@George: Good job, +2 :) –  j_random_hacker Jan 31 '11 at 1:03

I guess it is:

Me no can get program working - you help please code here pls debug. Thxxxx

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Apparently he/she has an assignment with a deadline :) –  Kenan F. Deen Jan 12 '11 at 21:17
Four years later, this answer is still a H O O T –  Howard Pautz Sep 12 at 5:53

On Server Fault, it's definitely "How do I host my server from home":

(And that's not even all of them.)

Which is ridiculous because the answer is always "Don't" - and I thought this was a site meant for professionals...

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Well, sometimes "don't" is the proper and professional answer... but that's a different discussion for a different time. I would have figured that was a prime target for duplicates, I just never expected that much ... variety ... in the wording. –  AnonJr May 17 '10 at 18:23
Don't is definitely not the answer, the answer comes in regards to availability guidelines. If you only LIKE to have your site up, be all means host from home. If you NEED your site up. Use a real host. –  Chris Marisic Jan 11 '11 at 19:03
What's scary is the same answer applies to "How to parse HTML with RegEx" - again the ideal answer is "Don't" but there are some limited cases where it's possible ;) –  Zhaph - Ben Duguid Apr 5 '12 at 9:52
As of today, a search for +host +server +home turns up 1,153 results on SF. Not all of them are this question, but it appears that the vast majority are. –  Michael Hampton Oct 4 '12 at 23:36
That looks more like a topic than a question archetype. –  Simon Kuang May 31 at 5:51

Since Jarrod spent so much time and effort cleansing the data, I figured it'd be a shame not to have some fun with it.

Here are the top 30 questions (by # of questions closed as duplicates of them) on SO:

572    How to fix "Headers already sent" error in PHP
525    mysql_fetch_array() expects parameter 1 to be resource, boolean given in select
405    How do I compare strings in Java?
306    Why are these constructs undefined behavior?
275    How do you parse and process HTML/XML in PHP?
268    How can I prevent SQL injection in PHP?
240    How to return the response from an AJAX call?
205    Reference - What does this symbol mean in PHP?
192    Reference - What does this error mean in PHP?
188    PHP: "Notice: Undefined variable" and "Notice: Undefined index"
160    android.os.NetworkOnMainThreadException
152    Is floating point math broken?
138    RegEx match open tags except XHTML self-contained tags
133    Why can templates only be implemented in the header file?
132    What is an undefined reference/unresolved external symbol error and how do I fix it?
121    Undefined Behavior and Sequence Points
116    "Least Astonishment" in Python: which scope is the Mutable Default Argument in?
115    The Definitive C++ Book Guide and List
101    What is a NullReferenceException and how do I fix it?
  99    Javascript closure inside loops - simple practical example
  98    UTF-8 all the way through
  91    Using a regular expression to validate an email address
  88    Is Java "pass-by-reference"?
  83    var functionName = function() {} vs function functionName() {}
  81    How can I convert ereg expressions to preg in PHP?
  80    How can I get query string values in JavaScript?
  78    Can a local variable's memory be accessed outside its scope?
  76    Access / process (nested) objects, arrays or JSON
  73    Call to a member function on a non-object
  70    Grabbing the href attribute of an A element

(* == closed; some of these are duplicates themselves, and have since been merged, some are just really persistent OT / NC)
(And yes, I'm counting deleted posts too - lookin' at you, PHP)

select top 30 count(*) as dups, 
 concat('http://stackoverflow.com/q/', RelatedPostId) as Q,
 case when q.ClosedDate is not null then '*' else '' end as Closed

from PostLinks
 join Posts q on q.Id=RelatedPostId

where LinkTypeId=3

group by RelatedPostId,q.ClosedDate
order by count(*) desc

This is now available for your use and abuse on the Stack Exchange Data Explorer!

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The MySQL one would have been my guess for the top-spot... they're a never-ending deluge; and those are just the ones that got closed as dupes of the merged dupes at the top... your query isn't recursive ;-). –  ben is uǝq backwards Feb 20 '13 at 8:43
The data code appears to be broken. I have posted a new version on my answer: meta.stackexchange.com/a/244636/200117 –  Ciro Santilli Dec 4 at 14:00

One I see often is "How do I center an element vertically and horizontally with CSS?"

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The worst part is this question has like 8500 answers all of which work in some ways and don't work in others. –  Chris Marisic Jan 11 '11 at 19:04

Attempting to modify a C string results in an access violation:

This must come up once a week.

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If not more .... –  Tim Post Jul 20 '10 at 10:44

I have a 3 pointer:

http://stackoverflow.com/questions/10644/any-decent-c-profilers-out-there/231577 http://stackoverflow.com/questions/49912/best-dotnet-memory-and-performance-profiler http://stackoverflow.com/questions/3927/what-are-some-good-net-profilers

Really they should all be folded into a single mega community wiki .... There is no value in having the 3 questions.

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Agreed, they should probably be merged. 3 points for you :) –  Jonik Aug 3 '09 at 20:36
nobody asking for an indecent profiler? –  Thorbjørn Ravn Andersen Jun 4 '11 at 11:23


That's a duplicate score too high to count.

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Eric, there is no clear unambiguous list for the moderator to work with. See how Marc was able to resolve the dupe I posted in seconds. Surely you could beat my measly score of 2. –  waffles Jul 26 '09 at 14:30

"foreach captured variable c#"

Jon kindly listed some of them here... and that was just in November; it will be longer now...

The problem with this language gotcha is that until you understand what is happening, you can't know what to look for... IMO, this is one of the good use-cases for tag-specific home pages, where we can (manually) add a FAQ for that tag (i.e. link to the canonical answer).

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Ampersand language operators. Actually might be worthwhile improving search so that it can look for ampersands.

For Ruby that would be "What's the meaning of &:name":

PHP: What does the & or =& operator mean?

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This is quite "rampant": Best Java 7 features

Pretty much exact duplicate of What new features in java 7 do you find most useful? (posted 9 months earlier), very close to What are you looking forward to in Java 7? (6 months earlier), and somewhat close to many others.

What also springs to mind are the countless variations on the theme "What is the best Python (web) framework?", with ever so subtle differences in emphasis and formulation. (Right now I'm not delving too deep into those, but take a look at python+frameworks, python+framework and python+web-frameworks.)

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+1, you get a score of 2 for this, I'm only giving points for exact dupes that can be merged, and are listed here :p –  waffles Jul 26 '09 at 14:32

Android has some serious flaws in its touch event model, and this has caused many people to ask how to fix the problem (the real answer seems to be "upgrade to Gingerbread", sadly):

I've marked most of these of dupes of the original, not sure if merging or what is the solution here. The bug report mentioned in 4342464 is the most correct answer IME.

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Shog9's Data Explorer query didn't seem to work for me, so I crafted one that did: http://data.stackexchange.com/stackoverflow/query/254033/rampant-duplicates-questions-with-the-most-duplicates

  l.RelatedPostId AS [Post Link],
  COUNT(l.PostId) AS DupCount
  PostLinks l
  l.LinkTypeId = 3
  DupCount DESC
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Concatenate row values into a single column is fairly common for SQL Server


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Can you create a list of actual merge candidates, so I can score it :p –  waffles Jul 27 '09 at 8:38

In the past:

[Why/When] Should I use xxxx Programming Language?

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I'm interested in Java, but every Java regular expression question I see seems to be "plz send me teh regex".

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I feel like mod_rewrite should be on this list but I'm too lazy to go look up a lot of duplicates. I'll mark it CW and someone who's motivated (or tomorrow when I'm bored I) can go look some up.

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Haven't got bored yet? :P –  Trufa Jan 30 '11 at 21:51
@trufa haha, totally forgot about it. Now you've reminded me ;) –  jcolebrand Jan 31 '11 at 13:52
@drachenstern hehe no worries but don't dare look at me, I'm the laziest bastard you will ever encounter :) –  Trufa Jan 31 '11 at 14:58
@trufa ~ My motto is that all programmers should be lazy. But not too lazy. (mainly in re: functions and reuse, etc. Copy-paste twice, but after that make a function and quit copy-pasting, too much effort to do so, making a function makes you lazier ... blah blah TL;DR ;] ) –  jcolebrand Jan 31 '11 at 15:30
@drachenstern codinghorror.com/blog/2005/08/… –  Trufa Jan 31 '11 at 15:55
@trufa ~ SEEEEEE!!!! ;) :D –  jcolebrand Jan 31 '11 at 16:17

I'm pretty sure the one I encounter the most is along the lines of "How come .1 + .1 + .1 doesn't equal .3?".

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Anything where the answer is dos2unix. The symptoms are varied and the clue quotient of the OPs is mostly, shall we say, chequered. In the end, by the time it's solved, it's obviously off-topic for SO, but you see some good programming and debugging coaching in the speculations leading up to that.

The examples I am finding are less spectacular than the best I've seen. These all contain literally dos2unix in the answer, but there will be hundreds more with different wording.

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Having voted to close more than 1600 questions, and succeeding in many of them, these are the two I found (anecdotal) most annoying (in number, yes) from C#/.NET world:

  1. Get property name and type using lambda expression

  2. And even more duplicated is this: Conditional operator assignment with Nullable<value> types?

  3. And even more: Pass An Instantiated System.Type as a Type Parameter for a Generic Class

That's just from my experience. The language designers can take a cue to find the most requested feature..

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