issues: 170259709

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
170259709 MDU6SXNzdWUxNzAyNTk3MDk= 956 Skip identical indexes with non-unique values in align? 1217238 closed 0     3 2016-08-09T20:14:38Z 2016-08-19T01:19:47Z 2016-08-19T01:19:47Z MEMBER      

Currently, when objects with with non-unique (duplicated) values in one of their indexes are passed to align, an error surfaces from pandas: InvalidIndexError: Reindexing only valid with uniquely valued Index objects

We could certainly give a more informative error here (see this complaint on StackOverflow), but a bigger issue is that this probably isn't strictly necessary. Instead, we could skip indexes for alignment if they are already equal. This is slightly less principled (a non-unique index may indicate something has gone wrong), but certainly more convenient and more inline with how pandas works (e.g., it even allows arithmetic between objects with non-unique indexes, which I believe does not work currently in xarray).

Currently, we do this as a special case when merging arrays and exactly one has labels (see _align_for_merge in https://github.com/pydata/xarray/pull/950). But we could probably do this in general, either by default or with a flag to enable it (or turn it off). This would then propagate to every xarray operation that uses align under the covers.

 
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