A multi-account is considered to be when a player accesses two separate accounts/characters. Multi Account Rules.
Having a character die however, and then creating another is not a multi account. And yes, you can have the same last name. We usually only decline new handles that are close/similar to the old that way we can avoid confusion.
Questions about multi accounts or status of multi accounts should be directed to firstname.lastname@example.org.
Two people having the same last name doesn't mean that they're multi-accounts. It simply means that they are related in-character. If you were to die and create a new character with the same last name then it would not be a multi, and would simply be your new character being related to your deceased one. So, yes you could die and create a new character with the same last name and it wouldn't be an illegal multi.