System Information
Operating system: Ubuntu 17.10
Graphics card: GTX 1080

Blender Version
Broken: blender2.8 23284e4dde5
Broken: master 786870e26f8

Short description of error
Undo when dealing with separate by loose parts ends up with duplicate objects

Exact steps for others to reproduce the error

1. Go into edit mode on the default cube
2. select everything
3. shift d, z 3,
4. p -> separate by loose parts
5. undo
6. exit edit mode
7. notice how there are multiple objects now and a duplicate cube on the bottom.