public <T> AsyncMutateInBuilder arrayAddUnique(String path, T value) {
        this.mutationSpecs.add(new MutationSpec(Mutation.ARRAY_ADD_UNIQUE, path, value));
        return this;
    }
public < T > AsyncMutateInBuilder arrayAddUnique ( String path , T value ) { this . mutationSpecs . add ( new MutationSpec ( Mutation . ARRAY_ADD_UNIQUE , path , value ) ) ; return this ; }
Insert a value in an existing array only if the value
isn't already contained in the array (by way of string comparison).

@param path the path to mutate in the JSON.
@param value the value to insert.