Add a post metadata if only the key and value does not exist
In a WordPress post, these multiple values exist for a custom metadata with the key “client”
client=>Andy client=>Johny client=>April
I want to add a new metadata only if its value does not exist,
client=>Andy will not be added because it already exists.
client=>Susan will be added because it does not exist
The post will now have these metadata values
client=>Andy client=>Johny client=>April client=>Susan