Is it a good idea to lay a second course of shingles over an existing shingled roof?
It is common to lay up to three layers (at least in my area), but if the roof has lifting shingles laying another layer will just lift the new shingles.  If the roof is bad, it is better and probably cheaper in the long run to remove the old shingles and lay the new ones fresh. 

If the roof is not that bad a second layer can definitely save money, just make sure you check your local codes before doing it.

Another concern can be weight. Shingles weigh a lot so the more layers you add the more weight your roof has to carry, if you live in an area with heavy show fall weight could become more of an issue.

Personaly I prefer to start fresh each time, but I save money by doing the demo myself.