I've been away awhile and I'm getting closer to being able to utilize my tier 11 weapon (Maniacal Essence). That being said I working on figuring out the best modification crystals to best enhance it but do not know if I attach duplicate crystals if their stats stack or remain the same?

Yes and no.

The percentages don't stack and are independent of each other for things that have a chance of activating (Blessing / Curses / Crits/freeze/etc).

The actual icons do stack - so if you equipped crystals that add Attack/Defense icons....those specific icons would stack. The chance of activiting the curse would not stack and the chance of activating are independent of each other. You can also only activate the respective curse once at a time - you can't have multiple of the same curse going.

Check out this topic - https://subeta.net/forums.php/read/881248/

It's not that they don't stack, its that the curses have diminishing returns. You can stack 4 Fire EOR mod on a single weapon, and you will have a higher chance, but its not just 24% (6%+6%+6%+6%), its going to end up being about 22% chance of activating a curse. This is because there is a chance that 2 of your mods try to activate on the same turn, but there isn't a benefit to that.

There is also some diminishing return on the fact that if your curse retriggers while the same curse is still active, you lose some of the benefit.

So generally it's better to put more types of crystals on the same weapon.

For example, for Maniacal Essense, I would probably put 2 Fire EORs, 1 Dark EOR and 1 Magma EoR, so that it stacks with the natural icons of the weapon.

If instead of the 2nd fire EOR you put a water EOR on there you lose more damage if the opponent blocks that type than the icons you lose from curse overlapping. You could also check each opponet your fightning and figure out an Icon that they do not block at all, and but that icon there, but thats a lot of work for very little benefit.

At the end of the day though, you could also argue that Parched Mods are just so OP, and increasing damage is so irrelevent in most cases that every weapon should just be loaded with as many parched baguettes as possible.

But a Max Damage ME would probably be 1 Dark + 1 Fire + 1 Magma + 1 repeat.

Mine just has parched though.

explained how the mods stack pretty well. It is advantageous to stack the EoR that compliment the weapon's base attack icons, but you also have to consider who you are fighting.

I have my ME modded with two parched since most opponents freeze, one dark EoR (cheapest one that stacks well) and one fire EoR