I agree that items should only go one on registry unless it's something you'll need several of (i.e. towels, wipes, shampoo, etc.).
It's a pet peeve of mine when there is more than one registry and similar items are on both...I wonder if the parents really want 2 strollers or 2 bath tubs, or if I buy it from one store, has someone else already bought it from another? I always skip those items that I see on both because I'm worried about giving a duplicate.
I had a friend that registered at BBB, BRU, and Target, and at her shower she ended up getting 2 or 3 of several items that she only wanted 1 of, because people were shopping at different stores.