System Information
Operating system: Linux-4.15.0-112-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.57

Blender Version
Broken: version: 2.83.5, branch: master, commit date: 2020-08-19 06:07, hash: rBc2b144df395f
also tried in 2.91 alpha same result
Worked: /

Short description of error
Mesh with active shape keys and multires is beenig sculpted upon and pressed undo. This after leaving sculpt mode corrupts mesh by duplicating Shape Keys effect. The only solution I have found is reloading file.

Exact steps for others to reproduce the error
attached a prepared file with a simple mesh with two active shape keys and multires.
These steps assume using this attached file:
-open attached file
-select object "cube"
-change to sculpt mode
-drag a line on the mesh with active brush
-undo sculpt change
-switch back to object mode
-as you can see the effect of key shapes has been doubled

attached blender file

Adding before and after images for comparison
before undo