I'm surprised this hasn't been suggested before (tho I've talked with Wilco about it).
Other tools allow someone to share to Facebook (for example) once per day and gain additional points.
In upViral, you only get points the first time something is shared.
This somewhat restricts some users who would like to share more often. Sure, some people do it anyway even tho they don't get points, but why not provide just a little more incentive?
I realize it would be more programming to store data for each user about the last date they shared and some logic to check that, but it might be worth it.
Not sure what the percentage of people this would apply to, but it might be enough to make a campaign go just a little more viral.