Shortly after iTunes came out, probably in 2001, I tackled "the project" of ripping my CD collection to MP3s. At the time, I was concerned about disk space, and so I skipped CDs that I rarely listened to, or in some cases I would rip just a track or two and leave the rest untouched.
Maybe you already see the problem, here. The whole point of having a music library is that you can go through periods where you don't listen to some pieces, but then later you can circle back to them and enjoy them anew. You play some tracks too often, causing a kind of burnout, but then you leave them alone for a time and they become enjoyable again.
This normal progression has coincided with the growth of digital music, especially iPods and the iTunes Store. I'm now much more comfortable thinking of my digital library as my 'main' music library, though I still always purchase a CD of anything I want to have long-term (no DRM restrictions, always available to re-rip in a future format if need be). I listen to music from the digital library almost exclusively; except for my car stereo from time to time, I can't think of the last CD player I used to listen to music on a physical CD.
Over the past year or so, I've found I've made a mistake 2 or 3 times. I'll think of an older song or an album that I haven't heard in a long time. It's not in my digital library, so I look into buying it. In at least a couple of cases, I bought a track on iTunes, pleased to only have to pay 99 cents for a track I haven't thought about in years.
You probably see where this is going. I discover later that I have the track, or the whole album, sitting in my physical CD collection. Since I don't peruse the physical CDs much, and during "the project" several years ago I didn't feel that it deserved ripping, the fact that it was there has escaped my mind.
Fortunately I haven't done this too much, but iTunes did get a few dollars from me for tracks I technically already own.