Short description of error
When moving identical islands that share vertices on the model to the same place on UV space Blender automatically merges those shared vertices.
This generates problems while editing UV (in other software too) and exports UVs like this to FBX, which makes importing to Mudbox fail (doesn't display correctly, and creates some artifacts on mesh).
Exact steps for others to reproduce the error
Open attached example blend:
You will see two identical islands, move one of them to exact position of the other (you can use incremental snap on X to do this here)
Islands merge into one.
What should happen:
Island should share the same UV space but stay disconnected.
What should happen on FBX export:
They also should be separated just sharing the same UV space.