I had the same problem as discussed at this resolved thread: Double Entry.
All my weblog entries were being displayed twice. This occurred after importing new members with a custom solution (in my case from phpList). The problem was caused somehow by having accidentally created a few double rows for some members in the exp_member_data and exp_member_homepage database tables. This occurred because some of our members (including me) had already joined our new EE site then had their records from the old site imported as well.
Deleting the double rows in those 2 tables fixed the problem. I am reporting this because this solution is much quicker than reinstalling EE as is implied at the Double Entry thread.
I thank carvingCode who reported the similar problem and Sue Crocker, Mark Bowen and Lisa Wess for working on it and especially Lisa who worked what the problem was. Your work saved me many hours!