Client group + promotions

[brosado]

Hello,

 

after searching a lot on google, here on forum and on modules i still can't find a solution for a small simple thing (i guess)

 

If i add a client to a 10% discount group, and if that client buys something from promotions, they get 2 discounts... one from promotion + "-10%"

 

My question is;

 

how to say the group don't sum "discounts + promotions" or simply to say that "promotions = 0% discount"

 

Thank you