I am seeing some odd behavior with accounts and the associated contacts. When I make a new account, it doesn't seem like the Unique ID for an account is just the name. I tried to make a new account in a different business unit, that had the same name as another account in the default business unit. When I tabbed down to the email field, it popped up a message asking if I wanted to use the email ID of the already created account, so I chose yes. Then hit save. I was testing to see if different business units could have unique sets of accounts and contacts, even if they are named the same. I received an error that the account already existed. I then just changed the name of the account, and it let me make it. However, it re-mapped all of my users to that named account and the original account was gone. It looks like that it just renamed the account and did not make a new one. This would prove that the name is not the unique identifier.

Reproduce

Create an account, the create some users and associate them to that account. Then attempt to make a new account and tab down to the email field and it should ask if you want to use the same info as the already created account. When you do this, choose yes, but then just change the name by a letter. It should then allow you to make the account. Then look at the list of contacts that you created, and you should see that the users are now belonging to the renamed account.