If you enter an email that to which you've already sent an invitation through our system, we will not send a second invitation. Instead, they'll get a different email that is a nice reminder message, a link to the box, a few of the latest examples of cards and the latest list of contributors. In this way, it helps build the social momentum.
You can see this list of people who have been invited by clicking on the arrow at the bottom of the header (just above the cards), which looks like this:
The screen will expand in accordion fashion and show you the list of invitees as well as who has made a card already.