BUG: Thoroughly dedup column names in read_csv
Caught after merging in #17060.

This is for when we specify the parameter `names=(...)`.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=h1) Report
> Merging [#17095](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/465c59f964c8d71d8bedd16fcaa00e4328177cb1?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17095/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17095      +/-   ##
==========================================
- Coverage   91.02%   91.01%   -0.02%     
==========================================
  Files         161      161              
  Lines       49397    49400       +3     
==========================================
- Hits        44966    44960       -6     
- Misses       4431     4440       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.78% <100%> (ø)` | :arrow_up: |
| #single | `40.28% <40%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.45% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=footer). Last update [465c59f...0aa97a7](https://codecov.io/gh/pandas-dev/pandas/pull/17095?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Everything is green.  PTAL. @jreback , @jorisvandenbossche : Given my comments <a href="https://github.com/pandas-dev/pandas/pull/17095#discussion_r130300173">above</a>, is there anything else that needs to be addressed in this PR? thanks @gfyoung 

can you do a followup to deprecate / error on ambiguous cases. let's try to make this simpler. 

pls make an issue about this for discussion / reference with an example. Sure thing.