What happened

One of my players was testing a dupe theory about throwing an item into the transistion layer and getting a second one on the other side.

What I did

That is all I know.

Used versions

Minecraft-1.12.2
VerticallyStackedDimensions-1.12.2-0.1.7.2.
CD4017BE_lib-1.12.2-6.4.17.1.
Forge-14.23.5.2847
other installed mods (only relevant ones, like those providing dimensions to your dimension stacks or modifying terrain generation)
Advent of Ascension

Crash Log:
crash-2020-03-13_21.39.42-server.txt

The new release should fix the crash issue.

And just in case you actually manage to find a working item duplication glitch, I would be very interested to know about it.