Examples of being strange about plural identities:
Treating fictives like they're your blorbo as opposed to a whole guy.
Deciding for yourself how a fictive or factive should relate to their source, if you think fictives & factives *must* be different from their source in some amount, you are not normal about plural identities. If you make moral judgements on an introject for the contents of their source material, you are not normal about plural identities.
Looking at spiritual aspects of plurality with undue skepticism and doubt when they don't apply to you.
Talking down to a fictive or factive for having a "problematic" source, and assuming that everyone gets to pick exactly who they are.
Refusing to use everyone's individual name when asked, or only recognizing one person in the collective.
Gatekeeping and fakeclaiming. You're not normal about plural identities if you insist all plural people need to have trauma. You're not normal about plural identities if you think someone has "too many alters" or perceive their conversation as "roleplaying".
Dictating what someone is or is not able to experience based on your personal understanding of their history.
Reacting to someone's plurality by imagining and feeling sorry for the trauma you assume they went through.
Finding the idea of plurality scary. Even past the evil alter stuff, if you think someone switching without you noticing is frightening, you are not normal about plural people. If you read or watch horror based around a killer with DID or a tulpa who kills its creator, you are probably not very normal about plural identities.
Any "eww" or "cringe" reaction to someone talking about themselves.
Dismissing experiences because you personally don't view them as valid or real.