There's two nameplates when I use Kui. Both the default nameplates and Kui. How do I fix this?

That implies that you have another nameplate add-on enabled. Can you post a screenshot?

I think he mean this... Got the problem too.

Cannadis18: You have another nameplate add-on enabled. I couldn't tell you which one it is, but you just need to disable it in your add-on list (or if it's a compilation like ElvUI, in its options interface)

i got urs and... maybe spartanui ?

Ya, it's SpartanUI. I had exactly the same problem and just looked and found that SpartanUI has added namplates. Click on Modules and de-select nameplates and kui will be the only one on.