different behavior in release/debug mode for future completion
https://github.com/status-im/nim-asyncdispatch2/blob/90f2a50dda7f084cfca1c64e402d484847514a07/asyncdispatch2/asyncfutures2.nim#L172

* documentation implies you can complete a future twice.
* release implementation allows you to complete a future twice.
* debug implementation does not.

multiple completions of the same future generally indicate issues, thus it seems the debug behavior is safer..