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 
Please do not use the `np.random` singleton in tests but rather a fix `RandomState` instance such as `np.random.RandomState(0)`.

Otherwise one would expect the latter confidence interval assertion to fail 5% of the time randomly which is not acceptable.

If the fit / predict is very fast to execute, this check can be run 100 times and the number of times the confidence interval check match can be counted and an assertion on that count can be added. It should be close to 95 times. The exact value can be set in the check as the random state should be fixed.