Hi Paul,
First, I am assuming you're talking about your Sales Insight watch list (not sure you meant to post this to Real Time Personalization)?
In regards to the MSI watch list, when you click the Add Account button, it's not really adding the account itself, but rather, all of the contacts you have associated with that account to your watch list. It shouldn't add duplicate contacts to your list.
Note that, to my knowledge, you can't "remove an account" the same way you can add. The only way I have seen is that you must remove contacts you no longer want in your list, individually.