"Concurrent refresh token requests were made by the same client" error?

[gravitas1]

"Concurrent refresh token requests were made by the same client for the same 

user, and while one of these requests probably succeeded, this one did not due to an update conflict. Visit https://dev.fitbit.com/docs/oauth2 for more information on the Fitbi

t Web API authorization process."

I thought we would be able to hit the refresh endpoint within a 2-minute window starting when we first use it?

[gravitas1]

Also, this error isn't documented anywhere in that link the error message provided. The 2-minute grace period was not documented anywhere and knowledge of that had to be obtained from the forums. 

[JeremiahFitbit · Accepted solution]

Your application will get this error if it makes multiple refresh token requests using the same refresh token within microseconds of each other. Please do not do this.

The 2 minute grace period is a safety feature that should not be relied upon. Instead, your application should make an earnest effort to only make one request using a given refresh token.