Version: 2.70 official (win 64)
How to reproduce:
- Link the composite node group in the provided .blend (or any node group)
- Add the linked node group into the composite node editor of another blend file.
- Perform some null actions, like moving a node around
- Press ctrl+Z to undo.
- Notice that each instance of undo adds 1 to the number of users of the linked node group. There is no reason for this.