Why would you want to create your own when the auto-enter serial works perfectly AND it will never break, which any user-created unique ID will break eventually.
Perhaps a better question for me to ask - how does one keep a person signing in to the database from repeatedly creating new records?
I want to check for an existing record for the individual attempting to sign -in.
Describe what you need in more detail.
Simply opening a database doesn't create a new record. You clearly have something more going on here, but don't tell us what that is.
If your records contain an account name in a field and you want to find that record, you can use field options to set a script to run that searches for a record with the current user's account name and creates a new record only if no such record is found.