Ideas on how to avoid multiple accounts for one user

I'm struggling with ways to avoid event registrants creating multiple accounts. For example when a member asks an admin person at their workplace to register an event for them and they then create a new account to register, rather than logging into the existing account. As a result the event does not show on the member event history in their correct account. How do we force people who have an existing account to login first...? Or alternatively, is there a way to merge their event registration with their correct account? Interested to see how people deal with this issue. Thanks

The question here is how do we know that they have existing account? When someone visit your site, there is no way the system know that they have existing account and force them to login

Even if they have existing account and they still register for new one, we don't have a way to force them to login first

So maybe for these registration records, you can edit the registration and choose the right user account for it?