Imagine that opponent have deck have full of "stop opp bonus" cards, like nightmares and you have 2 Leader card in your hand.
Leader bonus = calcel Leader
lets imagine more, your leaders are Ambre (courage 3) and Hugo (attack +6), both in your hand.
You play Ambre with the bonus "cancel Leader" and opponents plays whatever with the bonus "stop opp bonus"
In my thinking, it stops ambres bonus "cancel Leader", so Hugo during the attack is not cancelled, so it should give the attack of +6 to Ambre.
Question is : does it work like that ?

No the leaders bonus can't be stopped by stop opp bonus

The first problem is having two Leader cards in your deck will cancel the bonus they would normally provide, so you wouldn't have Amber's courage 3 or Hugo's attack +6.

Exactly...

leaders bonus and abilities can't be stopped by stop opp ability or bonus...

What if you have 2 of the same leaders in your hand?

for example you have Ambre and Ambre ........

will they cancel Leader or will they give (gasp) Courage 6 ?

2 of the same Leader cancels their Team ability. Simple rule, One Leader in your deck (unless you have a deck of 100+ cards to level up and you have multiple Vansaar).

They cancel too.

Leader are the only acception to the same card Bonus rule i tested the theory a long while back, hoping that i could power level Vansaars...
but, as for your scenario, you fail to realize that Hugo also has Cancel Leader, so, even though Ambre Bonus is canceled, Hugo bonus will still take affect. therefore still canceling Leader abilities.