So, I have a legendary melee weapon with the wounding prefix that causes targets to bleed for an additional 25 damage. My question is if I were to say give the weapon a serrated edge for bleeding damage does that mean it would do double the bleeding damage, or would it just do the one and not the other?

Depends whether there is a mod "sitting" on the slot you wanna mod.

Does it say something like "no misc" or standard or is the legendary mod atztached to the same slot?

If not the damage should add, but easiest would be just to quicksave and try?

One of the options for the sword is a serrated edge to add bleed damage, but as it's legendary prefix is wounding it already has bleed damage. SO what I am getting at is if I were to add the serrated edge would it still do 25 bleed damage, or would it do 50?

I'll just have to play around with it and report back when I have an idea.