How do keep duplicates from developing in my contacts, and then how do clean all the duplicates?

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

If your contacts are stored in iCloud then....

Remove duplicate contacts

1. Make a copy of your contacts.
2. Open Contacts.
3. From the Card menu, select Card > Look for Duplicates.
4. When asked, click Merge.
5. Repeat steps 2 and 3 until no duplicates are found.
6. Make another copy of your iCloud Contacts.

If you see duplicate contacts after setting up iCloud Contacts ...