Note that a PXEServerSignOn should only occur when the PXE server first connects to the engine.
Other symptoms of this is PXE-E53 and PXE clients not selecting the proper PXE automation option when automation jobs are assigned to the clients. Engine crashing may also occur if there are multiple PXE servers constantly refreshing their connection to the engine.
For DS 6.9 you need to either manually add unique data to the serial/UUID fields of those computers in the database or you need to delete those computers from Deployment Solution and not let them connect back up.
To find which computers are causing the problem run the following SQL query.
WHERE serial_num IS NULL
An alternate resolution would be to use MAC only as the primary look up key instead of SERIAL or UUID or a hybrid that uses those.
This issue has been resolved within Deployment Solution 6.9 SP1.
Any machine records that contain a NULL Primary lookup key field will not be sent to the PXE Server. The PXE Server will treat computers with a NULL Primary lookup key in the same manner as an unknown machine.
Deployment Solution 6.9
Clients inventory missing serial numbers or UUIDs.