Today I jumped on stack exchange to find I've been awarded the Security Exchange Yearling achievement that I was already awarded yesterday.
I assume this is a bug?
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No, it's no bug. One of the badges is grey, which means it's a badge for the Meta site.
Check your badge page and you'll see one for main, one for Meta.
You'll get one badge per side of the fence each time you earn more than 200 reps. So if you don't post so much on the Meta site, you might get less Meta Yearling badges than main site Yearling badges.