The same design number on different parts?

[Magnus Forsberg]

Following the decision of how to name the stud on the minifig heads 3626a/b/c,
I've found that the same design number is used on more than one part.

We have at the PT six heads with numbers (part alias):
10756, 46506, 86294, 89510, 90787 and 99570

A search on those numbers at Brickset shows cgi-pictures of both 3626b an 3626c heads;
10756 (design number), leads to 6006387, (item number) and shows a 3026c-head.
46506, (4200908) shows a b-head.
86294, (4549621) shows a b-head.
89510, (4578503) shows a b-head.
90787, (4584727) shows a b-head.
99570, (4651443) shows a c-head.

A search at Bricklink of the item-numbers leads to both 3626b and 3626c heads;
6006387, leads to 3626cpb719 (and a reference to 3626bpb719 as a similar design)
4200908, leads to 3626bpb122 AND 3626cpb122
4549621, 3626bpb366 AND 3626cpb366
4578503, 3626bpb332 AND 3626cpb332
4584727, 3626bpb471 AND 3626cpb471
4651443, 3626bpb633 AND 3626cpb633

Are TLG using the same design and item numbers on more than one part?
How do we handle that in our library?