Firebase prevent duplicate categories using rule [duplicate]

Answer 1

As per suggested by Frank van Puffelen

I have changed my data structure as following

and also updated Firebase rules as following

{
 "rules": {
   ".read": "auth != null",
   ".write": "auth != null",

 "categories": {
   "$categorie":{
        ".validate": "!data.exists()"
   }
 }
}
}

Now it's not inserting data if data is already there.

Thanks Frank van Puffelen