Combining an Absorption Hopper and EXP Tap in close proximity will cause EXP to duplicate.

You can recreate it by having an Absorption Hopper output its EXP into a Singularity Tank, and have that tank output EXP through a EXP Tap. Walk away, which will cause the EXP to go back into the Absorption Hopper, creating duplication. Walk back to it and enjoy your infinite EXP.

so far ive not been able to reproduce this, are you sure its not just picking up the xp from the mobs dieing to the alter i see right next to it?

mod version?

mod version?

According to the All The Mods 6 modlist, mob_grinding_utils-1.16.5-0.4.23.jar

so far ive not been able to reproduce this, are you sure its not just picking up the xp from the mobs dieing to the alter i see right next to it?

I'm very sure. Here's a video. It's possible it's because I am on a server, but I have set the difficulty to peaceful and the duplication still occurs. https://youtu.be/uIaFzvZim9g

2nd Confirmation.

I can confirm this as I just checked to add it to the list of issues.
1.16.5 (sky factory one 1.0.1).
Java 14
Ubuntu 20 LTS
Afaict, The tap drops xp orbs, which get absorbed by the player and the absorption hopper causing the duplication. Also, This processes seems to be (can be) additive, the tank will fill by itself.

I also installed the xp clumping mod to reduce lag, not sure if that's possibly causing the issue and why it's intermittent.

It is indeed an interaction with clumps.