Ok so I have a ring a necklace and some gauntlets all with an enchantment making bows do 40% more damage. Do they all add up as in bows now do 120% more damage or am I just wasting an enchantment? Thanks.

I don't see why they wouldn't stack.
To test you can equip one then check the damage of the bow,then equip another one to see if the damage goes up.
Also the damage of your arrows should go up,at least they do with fortify marksmen potions.

All enchantments and abilities(like the racial frost resistance of the nords) stack with one another.