I'm not entirely sure this is coroutil or zombie awareness' doing but it's the only 2 mods I added to my server before spawning went wonky that actually do anything with spawning.

Forge ver: 14.23.4.2730
Coro util: 1.2.11
ZombAware: 1.11.14

Extra utilities 2: 1.8.1

Extra utilities cursed ground was used to make effectively a mob farm and when installing these mods it made the weirdest thing happen.

When killing an enemy (or possibly just them spawning, needs testing) 3 more of that enemy will spawn behind the player at random.
This means if you kill a creeper 3 can spawns right behind you only blocks away which is kinda bad for my server theme(being fairness)
If you kill a skeleton then 3 may spawn right inside your house with you... For the love of all things I hope this doesn't apply to lesser shoggoths...

What's going on here?

So strange, Cojo had this issue in a previous server but I couldn't make sense of it, he confirmed its exactly the same issue from his memory. A really weird bug, all I can suggest right now is we try and turn off some things that might be misbehaving.

In config/ZombieAwareness/Features.cfg, set zombieSummonHelpBaseChance to -1 to make it not do anything, though you might want to also reset forges config files too. I turn off the vanilla feature of zombies "calling for help" which spawns another zombie near you, but maybe my turning off of it is misbehaving so lets try disabling that.

The only other default on feature I have that spawns extra zombies is the 'maxZombiesNight' in Spawning.cfg, turn it to 0, though it should only do its thing if zombies are far away but lets play it safe and turn it off.

With all that changed, let me know if the issue persists.

I'm pretty sure that was it, havent had any cloning of mobs since changing the config

It might've been bionisation3's cloning bacteria, as every mob that spawns has a 1 in 10 of getting it. Switched the setting to 1 in 100 and will see if that does anything. I figured it might not be even related to this mod as it does nothing to mobs other than zombies, skeletons, and some spiders.