It's usually only once per title...but may be different if one is standard def and the other is a blu ray or other special edition (such as when the diamond editions are released).
only if you're updating, from dvd to blu ray. It's kind of a bummer, especially those who buy two copies like for home and one for the car. If you spent the money on a second copy, why shouldn't you get the points?
For any movie title you've entered into DMR, you can order a replacement disc only for a fraction of the cost. I believe it's about $5 per DVD and $8 per blu ray. Double check the web site, but I got the DMR email a few weeks ago.