This is not garantied to induce a copy:
<pre>
In [11]: a = np.zeros((10, 10, 10))

In [12]: af = np.identity(4)

In [13]: niimg = nibabel.Nifti1Image(a, af)

In [16]: data = niimg.get_data()

In [17]: data2 = niimg.get_data()

In [18]: np.may_share_memory(data, data2)
Out[18]: True
</pre>

Of course, if the input niimg was a file name this copy is irrelevant. This is why I think that it is of high priority to add a "copy" keyword argument to check_niimg.