cellpairs break for geometry with non-unique cell entries

[shakisparki]

Consider a geometry with multiple faces/cells having the same tuple of indices in the vertex buffer and are to be kept as separate faces.

A call to cellpairs at an edge having the same indices of vertices as a separate edge, will find more cells than intended (the total of the cells attached to all edges having the same indices of vertices), leading to an index out-of-bounds error.