> Unless I somehow push that switch button "awkwardly" then it
> may get triggered twice (or at least it looks like it was twice)

It might help if you clear INT0IF right before you leave the ISR,
if you aren't already doing that. The flag could get set by contact
noise during the ISR, and when GIE is re-enabled with RETFIE
as you leave, PC will jump straight back into the ISR. If you
have a short-ish ISR and RC debouncing this may not be the
case, but that might depend on the quality of the switch

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