We have received the following statement from a Microsoft spokesperson regarding this issue:

Microsoft is aware of an issue affecting Gamerscore for a small number of users on Xbox Live. As part of our investigation, we can confirm that no achievement data has been lost. We are releasing a fix shortly which will correct users’ Gamerscore to show the accurate total the next time they log into Xbox Live.

Over the past few days, we've noticed thousands of instances of Xbox duplicating achievement wins in certain players' games. In some cases, it's abundantly clear that there's a problem as the gamer has an impossible number of achievement wins and the Gamerscore unlocked is more than the maximum for the game.This also has a knock-on effect for the gamer's total Gamerscore — it is showing an incorrect total compared to the sum of the achievements they've unlocked.This is a reasonably widespread problem — out of 143,368 gamers we have scanned at the time of writing so far today, over 6% are currently showing an incorrect Gamerscore total.We've sent over our findings to our contacts at Xbox and will update this story as soon as we have some idea of when a fix might be in place.