If I import an e-mail list that contains someone who is already a subscriber, will they be added twice?

No they will not. When an e-mail is already entered into the subscriber list on SIDEARM, adding them a second time via an Excel import, will just add them to this new list. 

Note: The subscriber's stats will not change in this scenario. So if they have elected to opt out or if their stat is removed, they will remain as such, even though they are added to the new e-mail list.