issues: 169405530

Sometimes I want to reindex a dimension along which I've concatenated several Datasets. Index labels will often repeat until I've performed this operation. This has worked without problems up to xarray 0.7.2, but in 0.8 I now receive this error:

``` python import xarray import numpy as np

ds = xarray.Dataset({'data': (['dim0', 'dim1'], np.empty((5,10)))}, coords={'dim0': [0, 1, 2, 0, 1], 'dim1': list(range(10))}) ds['dim0'] = list(range(5)) ```

``` InvalidIndexError Traceback (most recent call last) <ipython-input-5-50e97aa6cb4e> in <module>() 4 ds = xarray.Dataset({'data': (['dim0', 'dim1'], np.empty((5,10)))}, 5 coords={'dim0': [0, 1, 2, 0, 1], 'dim1': list(range(10))}) ----> 6 ds['dim0'] = list(range(5))

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/dataset.py in setitem(self, key, value) 536 raise NotImplementedError('cannot yet use a dictionary as a key ' 537 'to set Dataset values') --> 538 self.update({key: value}) 539 540 def delitem(self, key):

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/dataset.py in update(self, other, inplace) 1434 dataset. 1435 """ -> 1436 variables, coord_names, dims = dataset_update_method(self, other) 1437 1438 return self._replace_vars_and_dims(variables, coord_names, dims,

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/merge.py in dataset_update_method(dataset, other) 490 priority_arg = 1 491 indexes = dataset.indexes --> 492 return merge_core(objs, priority_arg=priority_arg, indexes=indexes)

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/merge.py in merge_core(objs, compat, join, priority_arg, explicit_coords, indexes) 371 372 coerced = coerce_pandas_values(objs) --> 373 aligned = deep_align(coerced, join=join, copy=False, indexes=indexes) 374 expanded = expand_variable_dicts(aligned) 375

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/alignment.py in deep_align(list_of_variable_maps, join, copy, indexes) 146 out.append(variables) 147 --> 148 aligned = partial_align(*targets, join=join, copy=copy, indexes=indexes) 149 150 for key, aligned_obj in zip(keys, aligned):

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/alignment.py in partial_align(objects, kwargs) 109 valid_indexers = dict((k, v) for k, v in joined_indexes.items() 110 if k in obj.dims) --> 111 result.append(obj.reindex(copy=copy, *valid_indexers)) 112 return tuple(result) 113

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/dataset.py in reindex(self, indexers, method, tolerance, copy, **kw_indexers) 1216 1217 variables = alignment.reindex_variables( -> 1218 self.variables, self.indexes, indexers, method, tolerance, copy=copy) 1219 return self._replace_vars_and_dims(variables) 1220

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/xarray/core/alignment.py in reindex_variables(variables, indexes, indexers, method, tolerance, copy) 218 target = utils.safe_cast_to_index(indexers[name]) 219 indexer = index.get_indexer(target, method=method, --> 220 **get_indexer_kwargs) 221 222 to_shape[name] = len(target)

/home/rotis/anaconda/envs/calphadpy3/lib/python3.5/site-packages/pandas/indexes/base.py in get_indexer(self, target, method, limit, tolerance) 2011 2012 if not self.is_unique: -> 2013 raise InvalidIndexError('Reindexing only valid with uniquely' 2014 ' valued Index objects') 2015

InvalidIndexError: Reindexing only valid with uniquely valued Index objects ```