Personally, I've never copied anything intentionally from a previously published MOC, and I see no need to do so. Others may, and I won't discourage it, but I hate the idea of copying anything in Lego.
Revise, revise, revise. Before I publish anything, I've already revised the MOC at least two or three times. Always try to reach a bit higher.
System is great, if used sparingly. Check out 75121 Imperial Death Trooper for an example of a set that does this well.