Case 1 - Only dominos enabled (9.0.29-classic)
Default cast bar is hidden and dominos cast bar shown

Case 2 - Dominos + Luna v4100 enabled
Default cast bar is shown in addition to dominos cast bar

Ingame screenshot

Curseforge thread on dominos addon page
See https://www.curseforge.com/wow/addons/dominos?comment=8832

Ingame player castbar settings

I believe this also happens on TBC

Thanks for your work and your help !

Please uncheck the castbar in Lunas "Hide Blizzard" options. This prevents Luna from interacting with the default castbar at all and Dominos can then hide it. Otherwise there might be this conflict.

Hey there,

Thanks for the help ! Unfortunately the checkbox was already unchecked

Checking it then unchecking it again does solve the issue temporarily, but reloading UI displays both cast bar again.
There may be a conflict indeed

Checking it and reloading does result in dominos cast bar only so will workaround with this on my side.