I'm not sure if I've ever bought a repeat. However, when I showed my friend some FQ that I bought the other day, she asked me 'Are you sure you don't already have that one at home?' I haven't made the time to look and see yet. But considering how my fabric is spilling out into an open bookshelf I have, I think it's time I made some rag quilts. I've got a lot of batting already pre-cut into 8" squares for rag quilts!
You could always take a quick look at your stash before you go out. Or you can do what I do on shop hops; bring your materials with you (even if it's just 1" squares of the material that you're planning to have in the quilt)!