Version Info

• Minecraft, 1.12.2
• Fusion, 1.0.3

Are you using OptiFine: Not using Optifine

Steps to Reproduce

1. Start up Minecraft with the mods listed in the log
2. A couple seconds after the Minecraft loading screen pops up, it crashes.

Tried starting it with Optifine, however it crashed with the same log.
Crash report (~/logs/latest.log)
latest.log

Seems like Fusion and some other mod you have are trying to change the same Minecraft method, leading to a crash.

You are using a lot of mods, could you narrow down the mods needed to reproduce the issue?

Tried disabling many mods, however I couldn't find the culprit. Though, while I was disabling some mods, it gave a different crash, now having to do something with JEI. The strange thing is, even after reenabling all the mods, it still gave this crash.
latest.log

After disabling a bunch of mods and then reenabling them once again, I figured out the culprit. It was VintageFix who is making this crash.
https://www.curseforge.com/minecraft/mc-mods/vintagefix

Seems both VintageFix and I have added mitigations since you reported this issue, so it should now be fixed by updating either VintageFix or Fusion.
Thank you for reporting the issue!