For example, if I have a 100 phys damage skill with 200% effectivness of added damage, would added fire damage support give 44 damage or 88?
Edit: nvm I just realized it would double dip with added phys sources, definely not a thing.
They are not. This is in fact the reason the wording of those was changed ages back to "gained as extra" instead of "added as" - previously there were no modifiers that distinguished added damage from base damage, so when we added those we had to make sure not to describe non-added damage as added.