This is a SPOILER if Ты haven't read the Книги yet!!

No because although twins share DNA it is not all identical that why fingerprints are different. It is actually еще common to only have one twin with a mental illness.

Well, mental illnesses can happen if something happened to Ты as an infant. If Ты get shaken, for instance, your brain gets mixed up and Ты can have a mental illnes from that.