To keep the same device entry in Google Play after a firmware flash, you must keep the "Android ID" intact.
This key is a unique identifier used by Google Play to address your device.
After each firmware flash, this unique identifier is re-generated by default, and this leads to a new device entry in Google Play (I have around 20 duplicate entries in my Play account->Parameters).
This Android ID is stored in the system database, and it is easy to change it.
First, you have to retrieve the Android ID you want to keep over the firmware flashes:
SELECT * FROM secure WHERE name = 'android_id';
The "SELECT" command will display the Android id, like this:
After a firmware flash, you just have to update this Android ID. Do that at early stage (before installing Google Applications for example), so that all applications will take into account the updated ID:
UPDATE secure SET value = '<your preferred ID>' WHERE name = 'android_id';
With <your preferred ID> the one you got initially with the SELECT command (i.e. 123456789abcdef0 in my example).
Just reboot to be sure that any running application gets the new ID.