Twice born star set is bugged.

Frawr

I am wearing 5 piece twice born set.

Earlier today I was wearing Armour Master.

While wearing armour master, I had the Serpent mundus stone.

Now, having put twice born back on, I am trying to pick up a 2nd mundus and replace the serpent.

Each time I click on a mundus stone, it is over-writing the 2nd mundus stone and leaving the serpent in place.

Bugged. It should replace the first mundus surely?

Apoxsee

Unequip the fifth piece, choose mundus, then apply fifth piece pick up second mundus. You need to over right the original if you want to change it.

phaneub17_ESO

You can always remove the mundus stone buff manually from your character sheet window.