Torment grenade luanchers

I been testing out this upgrade with my raiders and I was thinking. Does each raider does a -1 to leadership or even if you got 3 raiders with around does it only do -1 leadership once. My local group is ok with the stacking but now I feel like I been doing this wrong

From the wording of the rule I'd say it doesn't stack as it describes the -1 penalty as arising from one or more vehicles with TGL.

Page 63 of our Codex to be exact.

Ok I just read it again and yea damn it would of been a great upgrade for assualt units