This happens because the IDs stored in the target machine are deleted by the re-image process and once that the target starts these properties are recreated and the server detect the target as a brand new one. More precisely the IDs are stored in the file tgt_info.properties (in the same folder where the log files are stored) and contains the TargetGUID, HWKey and the TargetID which are used by the server to identify the target when it post any update.
So you have two options:
Prevent the re-image process from deleting this file. For example you can save it and re-deploy it after the re-image. This is quite dependent on how the re-image process is performed .
Delete all the target with an old update time.
Starting from Remote Control 9.1.4.0309 you can filter the “offline targets”.
You can specify in the filter the number of days since last update time.
Once you get the list of the “not-reporting” targets click on the “All” checkbox.
then click on “Delete target”.
I hope that this will help you.