I've had my Garmin Vivoactive 3 for seven months. Suddenly, yesterday - after a reset - it began doubling my daily active calories. A 7km run that the day before registered as 419 calorie burn is today 896 calories. Same route, same effort. This started happening after I reset my device and removed it from the Connect app, then reconnected it in order to fix an issue I had with downloading new music from Spotify. This is very annoying because it worked perfectly before. I'm loathe to reset again in case something else goes wrong.
Update: this fixed the issue - https://support.garmin.com/en-US/?faq=wXawOFiAPK8cyIqwOEN88A&productID=73207&searchQuery=Activity%20class&tab=topics
Garmin Support got back to me within two days with this solution.