title: Dimension coordinates has fill values and hence, values are repeating
body: <p>I received some data files (NetCDF) from a colleague and I am trying to concatenate these files into a single file, so that I am able to run it with some of my previous scripts. The issue is that the NetCDF files I received have dimension "height" with fill values of 1e20. This is causing an error when I try almost any xarray operations on the files, because there are duplicate values in the dimension coordinates. </p>
  
  <p>Here is the info on one of the files (you can see the repeating values in the final values of the "height" coordinates): </p>
  
  <pre><code>&lt;xarray.Dataset&gt; Dimensions:  (height: 1061) 
  Coordinates:   * height   (height) float64 0.0 10.0 20.0 30.0 ... 1e+20 1e+20 1e+20 1e+20 
  Data variables:
              pres     (height) float32 ...
              tdry     (height) float32 ...
              rh       (height) float32 ...
              u_wind   (height) float32 ...
              v_wind   (height) float32 ...
              mr       (height) float32 ...
              theta    (height) float32 ...
              theta_e  (height) float32 ...
              theta_v  (height) float32 ...
              lat      (height) float32 ...
              lon      (height) float32 ...
              alt      (height) float32 ...
              wdir     (height) float32 ...
  </code></pre>
  
  <p>I thought may be reindexing to a new index with no fill values might help, but I am also unable to do that as the error pops up again that:</p>
  
  <pre><code>ValueError: cannot reindex or align along dimension 'height' because the index has duplicate values
  </code></pre>
  
  <p>If someone could help me out here, it would be a great favour! I have been struggling with this for a while, but maybe the solution is quite simple and my beginner's status is not really being helpful here. :/</p>