How do I determine whether a specific item has appeared in a list multiple times?

Are you saving anything more than the count of how many times they entered their name, like date/time info?

If not, then you could pull in a TinyDB instance with a NameSpace LoginCount using names as tags and login counts as values (0 if not found). Call it tdbLoginCount to separate it from any other TinyDBs you might keep, like tdbEmailAddress.