Typo fix

theano/tensor/basic.py

@@ -5073,7 +5073,7 @@ def inc_subtensor(x, y, inplace=False, set_instead_of_inc=False,
         if (x.broadcastable[dim + dim_offset]
                 and not y.broadcastable[dim]):
             # It is acceptable to try to increment a subtensor with a
-            # a broadcastable dim with a tensor that is not broadcastable
+            # broadcastable dim with a tensor that is not broadcastable
             # on that dimension. However, its length must then be 1.
             # We insert a Rebroadcast Op to make sure it is the case.
             y = addbroadcast(y, dim)