title: Remove duplicates in pandas time series
body: <p>I have a csv file with a time series that has the structure: <code>col1: date</code> <code>col2: value</code>. The csv file has a date from, say, Jan 1st to  April 30. I then have a second csv file with the difference that the date is Feb 1st until May 31. The values in the second column from February 1st until April 30 are the same in the first and second file. The same problem for a third csv file (March 1st until June 30), fourth etc.: same overalpping structure. I want to read these csv files but retain only unique dates from Jan 1st until, say, December 31 without repetitions in values. Is there a fast way to do this with Pandas dataframes?</p>