Who's teffilin & mezuzohs did the Sofer return to you?

UPDATE: It is more common than most think.

Who's tefilin & mezuzohs did  the Sofer return to you?

It's all too common for a Sofer to receive from numerous people their teffilin & or Mezuzohs to check at the same time.

The tefilin & or Mezuzohs when opened contain many different individual items from numerous different people, all of looks basically the same.

Do they get mixed up? Yes!
You paid a lot of money for your teffilin& Mezuzohs, now you got someone else's.

What system does the sofer have to prevent even the slightest chance of a mix up? Verify 100%