how do you stop duplicate contacts

Read the apple support article Get rid of duplicate contacts on iPhone - Apple Support and fix it.

You may have stored the same contacts in multiple email accounts.

Either remove the contacts from the rarely used email account or disable contacts from the rarely used mail account in the iOS Mail app.

Unlike some Android devices, contacts are neither stored locally on an iPhone nor in SIM. They are synced to an email account that you had Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other