You get two accounts when you log in with two different email addresses, for example once with your email and once with Apple's Hide My Email or a Google account with another address. Accounts can't be merged, so keep the one with your bookings, memberships and wallets, and delete the other.
Check which account to keep
Log in to each account and look at the Profile tab:
Upcoming for bookings.
Memberships.
Wallets for club wallet balances.
Leagues.
Keep the account that has these. Nothing is moved when you delete the other one.
Delete the account you don't need
Log in to the account you want to delete.
Open the Profile tab.
Scroll to the bottom and tap Settings.
Under Support, tap Delete account.
Tap Delete.
Log in to the account you want to keep.
Avoid it happening again
Always log in the same way, with the same email address.
With Apple, choose Share My Email rather than Hide My Email.
Who can help
If both accounts have bookings, memberships or wallets you need, contact your club and Nettla support before deleting anything.