Ok, I figured out what's going on here.
It seems that there is a "time constraint" in this plugin. It seems that 10 minutes or so have to pass by before you can sign again the same profile.

Hope this post helps other people that where thinking the same.

Thanks for the feedback everyone (though it was weeks ago!). I ran a couple more tests tonight, and it seems that you can sign another user's book as many times as you want with no time constraints at all. All you have to do to do so is logout and back in again.

-Aaron