Industrial Foregoing Version: 1.12.8-232
TeslaCoreLib Version: 1.0.15
The Mob Duplicator cannot spawn wither skeletons in the overworld (didn't test in other worlds). I have given it 5x5x4(height) space and tried to put the duplicator 1block, 2 blocks, 3 blocks above ground, doesn't work in any of the ocasions. It seems to consume essence and rf, but no skeletons spawned.

The problem lied somewhere else. The duplicator was trying to spawn the skeleton on top of it, however it couldn't because of the enderio conduits I guess, cuz the moment I put the underground, it started working. However I don't know why it was able to spawn evocation villagers (my bad not invokers, too much dota). Maybe when it's bigger than 2 blocks something prevents it from doing that?

Aaand just found why.... I didn't put any upgrades for the working are, sorry for wasting your time, it was just trying to spawn the wither skeleton in the 2 block are that is assigned, but due to the conduits it couldn't do it I guess.

Show me a picture of your setup

It works with other mobs, mainly invoker, cuz that's what I use it for

Have you tried having a 4 blocks high space?

I just did, and it spawned it on top ....

The mob duplicator checks if there is enough space for a mob to spawn, make sure there is enough for it.

How comes there is enough for an Invoker but not a Wither skeleton?

Wither skeletons are taller than invoker/villager