I think it stacks up, but poison doesn't you can't be poisoned by 2 cards, the second card played cancels the first poison.
One question regarding Leaders.
The bonuses they give (eg courage, attack etc) are regarded as ability, bonus or none of them?
I mean, can they be countered by abilities such as stop opponent ability or stop opponent bonus (for each card ofc)?
Yup,I also had that question in mind, but I think that the leaders' accumulated/added abilities to it's team cannot be stopped by (stop opp ability; ex. Striker)..why? because when you face Ashigaru it is unavoidable that you go first; you cannot stop Morphun's pill making ability at the end of turns; and you can't stop Bridget in giving life at the end of turns unlike..stop opp ability can't do that..so I think it is safe to say that it would only be fair for the other leaders to be like the above mentioned..the only thing I know of that can stop them is the leaders' own bonus which is "CANCEL Leader"..
But..if you really want confirmation..try taking a hit from Timber or any card from your opponent with Timber around while using "Dieter"..Dieter both has "stop opp ability and stop opp bonus" so we'll know if Dieter can nullify the leader's added effect ( 1 damage) then do the math..check how many damage you received to see if there was a ( 1 damage) or none..Gg's