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,
Result wanted: 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