Migration Question about multiple domains on 365

Hi everyone!

I have 2 domains on my 365.  (lets say Domain1 and Domain2).

I already completed the migration for Domain1.

However, some of the users have mailboxes in BOTH domains.

So, if I have already created John.Doe@Domain1.com... and I do a cutover migration for Domain2.... will it overwrite John.Doe@Domain1?  (Because they have the same prefixes and names).

Or will it actually prompt me for which domain I want to migrate all of the Domain2 mailboxes to?

If Domain1's mailboxes get overwritten, it will be complete chaos.

Thanks!

Shawn.