numpy.ma.MaskedArray.unshare_mask
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Copy the mask and set the sharedmask flag to False. Whether the mask is shared between masked arrays can be seen from the sharedmaskproperty.unshare_maskensures the mask is not shared. A copy of the mask is only made if it was shared.See also 
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    https://docs.scipy.org/doc/numpy-1.10.1/reference/generated/numpy.ma.MaskedArray.unshare_mask.html