I have duplicate contacts after transferring to a new my iPhone 7 from a 6s.

If you sync your contacts with iCloud and you have access to a computer you could sign into iCloud dot com with a browser, go to Contacts then select each duplicate by command clicking, then when they are all selected go to the Settings icon (cogwheel) bottom left and select “Delete”.

If you have a Mac and again you are syncing with iCloud you could go to the Contacts App and search for Duplicates and if found Merge them.

If you don’t have access to a computer then I believe you have to do it “longhand” in iOS or use a third party app to do the leg work, but I don’t have any experience of the latter I’m afraid.