In that case the obvious solution is a reference qualifier, of course, but from the p.o.v. of the system, those will be two (or more) separate users.
Even if their name is the same.
That means that if one filters out the "wrong" user, the correct user will not be able to access stuff meant for him, because it is registered for the "other" user (account) of his.
Unless the user can log in using any of his account, but in that case, if stuff is registered for multiple accounts of the same user, that user will have to constantly log of and back in to access the needed stuff.
For a precise answer, and to avoid the unpleasant situation described above. you need to specify not only that one e-mail address should be shown, but also which of the many and what is the logic of determining which is the one to include.
Depending on the use case, the unpleasant situation describe above could be mitigated by setting up delegation for the multiple accounts of a user - but again, more details are required to be able to tell.
