Only if the movie's a favorite of mine will I double-dip, or sometimes they get extremely cheap because nobody double-dipped it.
I found the special edition of Hellraiser for like $6 because when it came out, nobody cared. You either had it already or didn't want it at all at this point. The extra interview wasn't worth $15.
Sometimes, different features offered and different cuts of the film make a movie worth owning two copies of it.
Donnie Darko, for example, has two very different cuts. The director's cut is nice for clarity and better explaining of the plot, not to mention the extra disc of features. The original cut is better due to how much it makes you think.
Daredevil, the director's cut was much better than the theatrical cut. But the theatrical cut has all the features. The director's cut only had an interview with the director explaining why he made the changes in the R-rated version.
I've been mostly double-dipping lately since I went to Blu-ray, taking advantage of some deals and get some favorites. I dread the day that I will be tempted to do my first triple dip. That will probably be Underworld (got the original DVD when it came out, then the extended edition, now I want it in hi-def).