This article explains why Phantom may prompt you to create a username when restoring your wallet, even if you already have one.
Cause
Phantom shows the profile associated with the first wallet you restore (by importing a recovery phrase or signing in with your Apple or Google email). If the first wallet you restore does not have a profile, Phantom prompts you to claim a username. Any other wallet you restore afterward will appear under that newly created profile, even if one of those wallets already has a username.
What you can do
Restore the wallet that already has your profile and username first. Then import any additional wallets or accounts.