Yes, you need to sign up each time so that the people running the challenge can keep track of badges and winners. If you want to participate, it really isn't that difficult to pop into the thread and post "Me too" so that they'll know you're on board. It prevents the organizers from having to go through a very long list of tracker entries for people who may have only used it once or twice to see what it was about but really have no long-term interest in being a Sourcerer.
The other sourcing challenge lasts for a month, and you have to sign up each time for that one also.