Getting New Free Champion Shards

I'm level 6 and very new to the game. I just got the option to pick 1 out of 3 different champions to unlock permanently. I tried seeing if I owned Garen already while I was in this selection screen to be safe, but there wasn't an option for it. I bought Garen anyway. Apparently I DID already own him, so I couldn't unlock him and had no choice but to turn his shard into BE.

Well, if I knew I already owned him, I wouldn't have picked him. Why did he appear as an option if I already had him in my collection? Is there a way to reverse this?