I have to admit I'm a bit of a pack rat for car parts. I currently have two trucks and two cars, one truck is being parted out, two cars are both getting new engines and new transmissions (well, rebuilt) and the second truck has a brand new engine (not rebuilt) just waiting for it with a rebuilt trans when it is needed. Then there's the plethora of boxes full of car parts and bolts and nuts and brackets and switches and wiring and ... you get the picture.
Hopefully, when the two cars are done and have their drivetrains installed I can throw out all the extra parts that I probably won't need anymore. See, that's my problem... I say "probably" and it stays because I just never know when I might need it. And that logic, as self-defeating as it may sound, has saved my butt a few times when I needed something that can no longer be found at a wrecking yard!
And my Tide/Cheer/etc. duplicates are all for old oil, coolant and such so I can take it all to the recycling center when they have their free weekend!