Because I don't know what other people's rdm are wearing, so I was curious if anything would overwrite or not.
(I was just trying to test it myself, but I asked here also.)
People already replied to you but I'm adding my 2 cents.
I'm not sure if this applies to every single debuff in-game but generally for enfeebling magic an enf spell of a higher tier overwrites the lower tier, but spells of the same tier do not overwrite each other, even if the second one cast has higher "potency" (wether the potency comes from Skill, Stats or Potency+) it still won't overwrite.
To get back to your frazzle example, if someone with ***gear casts Frazzle 3 and lands it, and then a second RDM with BiS gear tries to land Frazzle 3 on the same target, it will say "no effect"
Slightly off topic but I learned over these boards a neat "trick" for Frazzle landing. Since the best gear requires some stats that imply a loss of pure macc, you might have issues landing Frazzle3 with BiS gear on some targets.
Create a specific set for Frazzle2, different from the "ideal" stats for frazzle, focusing exclusively on Macc.
Cast Frazzle2 on the target, even if the spell will have lower potency, it will have more chances to land.
Once that has landed the target will have a good amount of Meva-, making it easier for you to land Frazzle3 in your BiS set (that likely has way less macc than your Frazzle2 set mentioned above).
I see other people just use two different sets for Frazzle3, a "macc" one and a "potency" one, but because of what we just said (Frazzle of the same tier doesn't overwrite each other) if you land Frazzle3 with ***potency gear, you won't be able to land Frazzle3 with full potency gear after, whereas you can do that if you do the Frazzle2>Frazzle3 trick.
So I dunno, ymmv but to me this is a pretty nice trick. Even though it costs some time (two spells being cast rather than one) and in situations where you have to apply a shitton of debuffs on the same target in the least possible time, I reckon that might be unconvienent in such situations.