title: R/zoo: index entries in ‘order.by’ are not unique
body: <p>I have a .csv file containing 4 columns of data against a column of dates/times at one-minute intervals. Some timestamps are missing, so I'm trying to generate the missing dates/times and assign them NA values in the Y columns. I have previously done this with other .csv files with exactly the same formatting, with no issues. The code is:</p>
  
  <pre><code># read the csv file
  har10 = read.csv(fpath, header=TRUE);
  
  # set date
  har10$HAR.TS&lt;-as.POSIXct(har10$HAR.TS,format="%y/%m/%d %H:%M")
  
  # convert to zoo
  df1.zoo&lt;-zoo(har10[,-1],har10[,1]) #set date to Index
  
  # merge and generate NAs
  df2 &lt;- merge(df1.zoo,zoo(,seq(start(df1.zoo),end(df1.zoo),by="min")), all=TRUE)
  
  # write zoo object to .csv file in Home directory
  write.zoo(df2, file = "har10fixed.csv", sep = ",")
  </code></pre>
  
  <p>My data looks like this (for an entire year, more or less) after conversion to POSIXct, which seems to go fine:</p>
  
  <pre><code>                    HAR.TS        C1       C2         C3        C4
  1      2010-01-01 00:00:00 -4390.659 5042.423 -2241.6344 -2368.762
  2      2010-01-01 00:01:00 -4391.711 5042.056 -2241.1796 -2366.725
  3      2010-01-01 00:02:00 -4390.354 5043.003 -2242.5493 -2368.786
  4      2010-01-01 00:03:00 -4390.337 5038.570 -2242.7653 -2371.289
  </code></pre>
  
  <p>When I the "convert to zoo" step I get the following error:</p>
  
  <pre><code> Warning message:
   In zoo(har10[, -1], har10[, 1]) :
     some methods for “zoo” objects do not work if the index entries in ‘order.by’ are not unique
  </code></pre>
  
  <p>I have checked for duplicated entries but get no results:</p>
  
  <pre><code>&gt; anyDuplicated(har10)
  [1] 0
  </code></pre>
  
  <p>Any ideas? I have no idea why I'm getting this error on this file, but it has worked for previous ones. Thanks!</p>
  
  <hr>
  
  <p>EDIT: Reproducable form:</p>
  
  <p>EDIT 2: Have to remove the data/code, sorry!</p>