Since I'm only 20 measly points away from Platinum, I went through some old GameCube games I had to see if I had anything I never redeemed before joining. I did. But apparently, the GameCube game codes are still registered to your account as you owning them, but they don't get you squat on coins. 
You can get around this by creating a sub-account for another family member, your dog, etc. 
Then any games you've already registered to your primary account, if you get a duplicate register it under the sub account. It will earn points, and the points still collect in one "pool".
Are you sure? Because I've tried to register used games, and the codes were already redeemed, and it clearly says you can only use a code once after you try.
Not the same *code*, the same *game*. For example, if you register Super Mario Galaxy under your account then you end up with another code later on from that game, you can't register it again. Unless you create a sub account. I may have misunderstood, but it sounded like that was the issue you had run into.
No, I had bought a used game, with the code insert still in it. And when I went to register the code, I found that it had already been redeemed by a previous owner.
And you can register games twice, with different codes, under the same account. Unless the difference was that one version was a Nintendo Selects version, and the other was the original release. I've registered two different versions of Super Mario Galaxy to my account.
