Refresh tokens can be invalidated/expired in these cases
Solved: Re: Invalid or expired refresh tokens - Fitbit Community
1. User revokes access to your application
2. The refresh token is used to obtain a new access token and new refresh token
3. The user goes through the Authorization process again and gets a new refresh token
(At any given time, there is only 1 valid refresh token.)
とりあえずえくすぱいあ〜てきなものなしかな?
