Allow multiple input feature with the same value for gaussian processes

Fix for issue #3162. Allows to have multiple input feature with the same values for Gaussian Processes 
As @jnothman said this name is confusing. Can you please give an example of invalid input? Also function names are [lower case in Python](http://legacy.python.org/dev/peps/pep-0008/). A better name would be `_has_repeated_samples(self, pairwise_distances)`.