Reminds me of Mission Impossible where there are “3 people with that email address”.
From a technical standpoint there need to be something that is different between account 1 and account 2, and you system that are supposed to authenticate the accounts need to get that unique identifier as part of the process otherwise you will have a severe security whole.
So then it depends on why you would ever want for one credential to be several accounts and here are some scenarios that are familiar.
First where you have one account but in that account you can act as another user. We used it for account managers to be able to “log in” as their clients account. They still only have one set of credentials but in the system they have an option to swap to another user associated with their account.
Another scenario is Blizzards battlenet. Where you have one credential but your display name can be the same as other users. (You have a unique battle tag # to differentiate between two different users and thay use different login credentials too). They also have the option for the same account to play in several regions which in reality will be similar to have multiple accounts as things you do in one region is independent of things you do in another in the specific application (game in this case).