Duplicate values in Firebase Realtime Database

MIT App Inventor Help
1

Hello Everybody
I have certain of users and each user has value should be stored to them in realtime Firebase using spinner
example
Tag 0919876543: "["300153","Ali ","driver"]"
allplatenumbers:"["25-7722","25-7721","16-519"]"

why I don't want is for two tags take one plate number
example tag 1 choose 25-7722
tag 2 can't choose it

enroll

2

Create a second list of "taken" plate numbers, add the taken plate number when a new user enrols. Create a third list of available plate numbers all plate numbers minus the taken plate numbers.
When a new user enrols, only show them a list of plate numbers they can use.