If a user is only assigned to one account and that account is added to a campaign, they will only receive that campaign's emails once. If this account is added to the same campaign again, this user will not receive the emails again.
If a user is assigned to multiple accounts, however, they can receive the campaign emails once for each account they are on. So, they could receive the campaign emails once for "Account A" and once for "Account B".
This is to prevent duplicate emails from going out to the same user for the same account.