so if im a hexblade warlock, and my weapon uses charisma for damage modifier at level 1, and im multiclassed into sorcerer, granting a charisma damage modifier to my spell at level 6.

then if i use green-flame blade which at level 7 deals both weapon and fire damage , would i add charisma once or twice for the weapon attack damage?

i think i remember something about multiple instances of the same stat arent added together, but i cant remember if it was for ability checks or attacks, and i havent been able to find it again since then.

Read the spell Green-flame blade. 

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.

This spell’s damage increases when you reach higher levels. At 5th level, the melee attack deals an extra 1d8 fire damage to the target, and the fire damage to the second creature increases to 1d8 + your spellcasting ability modifier. Both damage rolls increase by 1d8 at 11th level and 17th level.

The target of your melee attack gets the normal melee attack damage (weapon+bonuses from stats and special abilities) and then the fire leaps from the first target and does fireamage equal to you spellcasting modifer (and 1d8 more at lvl 5 etc etc).

Your attack would deal:

Weapon: the weapon damage die plus your Charisma modifier
Spell: 1d8 fire damage from the Green Flame Blade spell.
Sorcery: additional fire damage equal to your Charisma modifier.

There is no rule that prevents an ability modifier applying twice if from different effects.

Okedoke thanks