Steps to reproduce

Create a content with two content types (A and B).
Each content type (A and B) inherit the same other content type (C).
Each content type have at least one group with a identical name (for example mygroup) with a part of the attributes come from the common content type (C).

Groups are merged but common attributes are removed. Only single attributes are kept. We should have attributes of the common content types once, they should not disappear.

If there is a real conflict (like an attribute with the same name in the two content types (A abd B) but not from the common content type (C)), it should failed for real (incompatible view or something like this).