Welcome to FT, Puregold98!
You can sign up for multiple airline/point versions of iDine as long as you use DIFFERENT credit cards. As you suspected, the same meal cannot earn points in more than one program--the most recent program you signed up for with that card will cancel out a previous registration. But you can sign up up to three cards per account, thus you can make more than one visit per month to a particular restaurant by paying with a different card each time.
Some personal advice--I don't consider Mypoints Dining a good value compared with a dining program affiliated with an airline like United. Look at the conversion rates:
Mypoints: One free air ticket = 40,000 pts/$4,000 spent on meals
United: One free air ticket = 25,000 miles/$2,500 spent on meals
[This message has been edited by TribeFlyer (edited 10-29-2003).]