Plugin is defining pluggable wp_mail() function. My idea was to check if function is defined already and throw warning if other plugin beat me to it.
However this warning causes issues on activation. As far as I understand during normal operation plugin is loaded before pluggable.php but for the purpose of activation check it is loaded after pluggables.
What would be the robust/proper/suggested way to implement such check for pluggable function? There is no obvious (for me) way to distinguish activation and handle it separately.
Don’t do the check on activation?
Seriously, the best way I can think of is not to check for this on activation, but only in the normal plugin load process. And instead of throwing a warning (I assume you mean a PHP E_WARNING), perhaps putting an admin error box up would make more sense.