title: resample or asfreq pandas time-series dataFrame in python error with 'Duplicate Index'
body: <p>I have a pandas dataframe where I have the datetime (not in the index, and preferred that way).
  I would like to up-sample (resample) that to a specified time-scale such as '10S'. And keep the string data (i.e. columns Acitivty/Action/EPIC etc) in the dataFrame as well.</p>
  
  <pre><code>Ind TIME_STAMP          Activity    Action  Quantity    EPIC   Price    Sub-activity    Venue   Position
  0   2018-08-22 08:01:36 Allocation  SELL    100.0       BB.    1.142200 CPTY     300AD  -427.0
  1   2018-08-22 08:02:17 Allocation  BUY     15.0        BB.    1.152300 CPTY    ZDDD02  -388.0
  2   2018-08-22 08:24:51 Allocation  SELL    60.0        BB.    1.165900 CPTY    666     -515.0
  3   2018-08-22 09:07:59 NaN         NaN     NaN         NaN    1.167921 NaN             -515.0
  4   2018-08-22 09:11:00 NaN         NaN     NaN         NaN    1.174500 NaN             
  </code></pre>
  
  <p>I have tried a few different methods, namely dataFrame.asfreq(freq = '10S'); and dataFrame.resample('10S', on ='TIME_STAMP')</p>
  
  <p>What I really would like to do, is to 
  1) up-sample the data into 10-second blocks, keeping the original data, 2) using the column 'TIME_STAMP'. and 3) after that, be able to fill in the numerical data using some fill method such as .fillna(method ='pad')</p>