Apparently there have been some mistakes made in the realm-first achievements being given out. You've probably seen these (Blizzard apparently broadcasts them across the realms) -- when someone hits level 80 or finishes a profession first, they attain a feat of strength achievement. They don't get the original titles, but they do get credit for getting their first.
But on a few realms, Blizzard ended up giving some of the awards to multiple players, resulting in the ingame mail you see above. We're not sure if it's because they all hit within a certain time period, or whether Blizzard's servers weren't working correctly, but as you can see, they say the issue is fixed for now. If you did rush to be the first on a server, you can be satisfied that you're the only one.