Mekanism adds their own version of obsidian dust as well, which can be created in their Enrichment Chamber, which creates 4 Obsidian Dust from one Obsidian.

Usually, you would then need 4 of those to create one Obsidian again, but Occultism adds a recipe which smelts 1 Obsidian Dust into one Obsidian, allowing to effectively duplicate Obsidian (at the cost of some energy, I suppose).

I believe a relatively simple fix for this would be to make mekanism and occultism Obsidian Dusts incompatible, so that mekanism Obsidian Dusts don't get caught by Occultism's recipe.

So this is not overly easy to fix - both mods use (and in my opinion) should use tags wherever possible. That means, any material that is added to this tag will be treated equally - usually this is good, as it enables compatibility.

In this case the only option would be to switch occultism or mekanism to not sue tags for obsidian and obsidian dust, which will cause trouble with other mods, depending on the exact combination of mods in a modpack.

Therefore, I think the real solution to this should be implemented on a per-modpack basis. Modpacks (using e.g. KubeJs) can modify either the recipe (e.g. setting the input in occultism to use only occultism dusts and create separate recipes for other mods with a 1:1 ratio between item and dust) or modify the tag (e.g. removing the mekanism dust from the obsidian dust tag, or creating multiple tags, one for each possible ratio).

The advantage of this is, that it can be adapted based on the needs of each modpack and the actual recipes used there - if there is another solution that has the same effect without breaking tags i'm happy to look into it though!

After looking into this (and your KubeJS) solution, I think you are absolutely right - the smelting back is the problem, and to be honest, I think I can remove this in general. I initially added it so you could reclaim your obsidian, but the utility of that is minimal, and it carries the risk of recipe weirdness with other crushing mods

Well, thanks for "fixing" this!

Hm, I can definitely try out kubejs to solve the issue for now. I wanted to test with some other mods that add obsidian dust or the likes but seems that every other such mod is 1.12 only.

I believe the crucial issue here is that it's possible to smelt one obsidian dust back to obsidian. Haven't invested too much into Occultism but it seems that obsidian dust not does need to be added to the ore dictionary, as it's not intended for mass-production or usage purposes, right?

Have tried KubeJS now, and

onEvent('recipes', event => {
    event.remove({output: 'minecraft:obsidian', input: 'mekanism:dust_obsidian'})
})

in startup_scripts/script.js seems to do exactly what I need.