If you really don't care about ruining any experience, that is. It's a cheap way to do it.
I'll actually say Colorless Souls are the only justifiable duping items. You can only get a maximum of 10 per playthrough, and have to be stuck at both Pure Black World and Character Tendencies to get them all. If you have multiple pieces of gear that need them for upgrading, you're looking at pushing past NG+s you may not want to be doing just for the souls, while being stuck at the hardest difficulty (making progression in getting those souls slower) and having being short on potentially important NPCs due to having to clear out a number of them for the Mephistopheles Colorless Soul reward. Basically, if you need more than 10, I'd say it's fine to dupe them.