Duplication glitch when moving cargo containers using EVA construction
Moving a cargo container to a different location on a vessel or dropping it on the ground will cause its cargo slots to appear multiple times in the EVA construction window. If any parts are in these slots then they will be duplicated. This can be reset by F5/F9 or going to the tracking station/KSC and back.
I have a couple of 1.25m cargo containers around my mining base. I use them for transporting things from orbit with my Orbital fuel hauler, which carries fuel from the ground to orbit, and it carries the containers on hardpoints so I can reove them anytime during EVA
Once I get the stuff in them, I land near the base and transfer the containers to hardpoints on my large tanker rover for transport to the actual base. It's easier than transferring one piece at a time or trying to carry multiple cargo containers for each ship.
I moved a bunch of deployed science stuff down there at one point and a lot of it got duplicated while moving the cargo containers around, so I set up my science stuff and still had tons of leftovers afterward (even though I brought no spare parts) and had to bring them all back home. These duplicates still exist in my game and I'm slowly bringing them home to get them out of the way. It also duplicated repair kits, jejtpack, and spare EVA fuel which were in there as well, which I'm also getting rid of in the name of fairness, since I never actually launched them.