Short description of error
Assuming there are n references to said group, when deleting a newly created object with an assigned dupli_group, after n times an error is thrown.
this can be fixed by saying Object.dupli_group = None before deleting.
Exact steps for others to reproduce the error
See attached blend file and run script.
There are 7 referenced instances of a group.
The script creates 10 new objects and assigns them a dupli_group value referring to this group. On the 8th, 9th and 10th, you get the error "ID user count error: GRGroup (from '[Main]')"