A sample of how my lipsticks and blushes are labeled. I just write on a post it in really small writing what the dupe is, cut it out and tape it on. I never cover up what the product's actual name or shade is.
All of my WNW 8 pan palette dupes are labeled with a sharpie. I wrote the names over the corresponding shade and then ran a swipe of Seche Vite nail polish top coat over top so it wouldn't rub off, lol.
Various labeling. We could go on for days, people. Usually all my dupes are labeled with a tiny piece of paper taped on the back of the product.
Hope this gives you some ideas on labeling your dupes! I highly recommend putting it right on your product, as it gets very hard to remember. And, if you just keep it scribbled in a notebook, you don't want to have to keep going back and looking it up. It takes a little time to do them all, but it really helps. Have a great week, peeps!