Well of souls possible mods

So I've searched and couldn't find the answer to my question.

Can some exclusive mods not be rolled if the item already has similar mods (ie: fire thorn on belts if the belt already has physical thorn or %phys damage+slower attack speed if the weapon already has %phys damage?

Either I am very unlucky or I am missing something as I have just wasted all of the currency I had gathered so far trying those 2 combinations (about 25 divines worth of currency using Omen of light to reroll failures and using the Kurgal/Amanamu guarantee mods and the echoes every time I tried. I have seen every possible Kurgal and Amanamu mods on the respective items, but the two I was looking for...

This is very discouraging...