It is located on line 792 of ARENA.LUA:
Code:
if (receiver_name == attacker_name) then
sdmg = sdmg*(1 + tonumber(hero_item_count("sp_branding_iron")/100))
end
What is weird is that the same units have to be attacking each other, which doesn't match your description.
I guess the description should say +20% damage when attacking an enemy unit that is the same as your unit.
Anyway, there you go.
/C\/C\