Hi,
Here is a weird example, on Ubuntu 17.04 with Python 3.5.3 and Numpy 1.13.0:

In [0]: import numpy as np
In [1]: set([np.nan] * 5)
Out[1]: {nan}                        # OK
In [2]: set(np.array([np.nan] * 5))    
Out[2]: {nan, nan, nan, nan, nan}    # <--- ???
In [3]: set([10] * 5)
Out[3]: {10}
In [4]: set(np.array([10] * 5))    
Out[4]: {10}                         # only for np.nan ??

I don't understand how {nan,nan,nan,nan,nan} is even possible. The array was constructed with np.nan replicated 5 times (and the same works with np.full(5, np.nan)).

I searched on StackOverflow and on the documentation, but cannot find any explanation for this weird behavior.
Thanks in advance if I'm just misunderstanding something!
But in case it's a bug, I can try to help (if a maintainer advise me on the good direction).