In SPO, we have a little problem, because when we invite an external user to our tenant, this person will receive an invitation, but he/she can decide whether use it or not, and instead of using it, forward the invitation to someone else to access the site or document.
So to prevent this, we can use the following PowerShell:
Set-SPOTenant -RequireAcceptingAccountMatchInvitedAccount $true
This will require that only the owner will have access 🙂
Cheers!
Albandrod's Memory 