In SysGen v3.1, a new library is created whenever the XtremeDSP hardware co-simulation flow is run. The new library stores the run-time co-simulation block. If a user generates the same subsystem twice, and doesn't close the library from the first run, the co-simulation flow will cause an error when it attempts to create the library (with the same name) during the second run.
This now causes an error message to be displayed, and the old library will be closed before the new library is put in its place.
This has been fixed in System Generator 3.1 service pack 1