This is a fallacy involving three sets: A, B and C. It is the assumption that because all A are members of C, and all B are members of C, that all A are B.
All A are members of C.
All B are members of C.
Therefore all A are B.
A. All cherry tomatoes are red things.
B. All cherries are red things.
C. Therefore all cherry tomatoes are cherries.
A. Fred is an atheist who lives in California.
B. Ted is also an atheist living in California.
C. Therefore Fed and Ted are actually the same person.
There is usually no reason to believe that two members of a group are identical or related, nor two subgroups of a group.