title: Resampling Error : cannot reindex a non-unique index with a method or limit
body: <p>I am using Pandas to structure and process Data.</p>
  
  <p>I have here a DataFrame with dates as index, Id and bitrate.
  I want to group my Data by Id and resample, at the same time, timedates which are relative to every Id, and finally keep the bitrate score.</p>
  
  <p>For example, given :</p>
  
  <pre><code>df = pd.DataFrame(
  {'Id' : ['CODI126640013.ts', 'CODI126622312.ts'],
  'beginning_time':['2016-07-08 02:17:42', '2016-07-08 02:05:35'], 
  'end_time' :['2016-07-08 02:17:55', '2016-07-08 02:26:11'],
  'bitrate': ['3750000', '3750000'],
  'type' : ['vod', 'catchup'],
  'unique_id' : ['f2514f6b-ce7e-4e1a-8f6a-3ac5d524be30', 'f2514f6b-ce7e-4e1a-8f6a-3ac5d524bb22']})
  </code></pre>
  
  <p>which gives :</p>
  
  <p><a href="https://i.stack.imgur.com/1g8sE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/1g8sE.png" alt="enter image description here"></a></p>
  
  <p>This is my code to get a unique column for dates with every time the Id and the bitrate :</p>
  
  <pre><code>df = df.drop(['type', 'unique_id'], axis=1)
  df.beginning_time = pd.to_datetime(df.beginning_time)
  df.end_time = pd.to_datetime(df.end_time)
  df = pd.melt(df, id_vars=['Id','bitrate'], value_name='dates').drop('variable', axis=1)
  df.set_index('dates', inplace=True)
  </code></pre>
  
  <p>which gives :</p>
  
  <p><a href="https://i.stack.imgur.com/HXi3M.png" rel="noreferrer"><img src="https://i.stack.imgur.com/HXi3M.png" alt="enter image description here"></a></p>
  
  <p>And now, time for Resample !
  This is my code :</p>
  
  <pre><code>print (df.groupby('Id').resample('1S').ffill())
  </code></pre>
  
  <p>And this is the result :</p>
  
  <p><a href="https://i.stack.imgur.com/KkAvd.png" rel="noreferrer"><img src="https://i.stack.imgur.com/KkAvd.png" alt="enter image description here"></a></p>
  
  <p>This is exactly what I want to do !
  I have 38279 logs with the same columns and I have an error message when I do the same thing. The first part works perfectly, and gives this :</p>
  
  <p><a href="https://i.stack.imgur.com/QsH0s.png" rel="noreferrer"><img src="https://i.stack.imgur.com/QsH0s.png" alt="enter image description here"></a></p>
  
  <p>The part <strong>(df.groupby('Id').resample('1S').ffill())</strong> gives this error message :</p>
  
  <pre><code>ValueError: cannot reindex a non-unique index with a method or limit
  </code></pre>
  
  <p>Any ideas ? Thnx !</p>