With Duplicate Remover you can find out any duplicated items of your inventory and remove them. Works with objects, textures, animations, sounds, gestures, landmarks, bodyparts, clothing, notecards & scripts.
Full help on Imitation Of Life Blog
See item in Second LifeWorks as well as can be expected, leaves some duplicates.
I was worried that it would delete items with the same name, but as it checks the UUID every time, it rather leaves duplicates of items that have been rezzed and then taken back into the inventory - which gives the object a new ID. It will remove direct copy-paste items, but for instance, if you have full perm animation packs that have some duplicates in them, checking the UUID will leave duplicates in place, as they'll have a new ID assigned when rezzing the pack.
I tested it by taking animations from the same creator's furniture, then dumping them all into the checker. It did not find one duplicate, even though the creator uses the same animations in many of her pieces of furniture.
As for landmarks, it works as well as for other items. Obviously, if you have a ton of them, the likelihood of it getting stuck is higher.
The script will help but not fix all of your problems for you. ;)
I would not recomend
I bought this product to tame my inventory. I tried it once by dumping my LM's in it and it broke. Delivered a few too me but then just died. I checked the sim performance thinking maybe it was SL but no...the sim was fine.