Add uniqueness for RunnerNamespace

app/models/ci/runner_namespace.rb

@@ -5,5 +5,7 @@ module Ci
     belongs_to :runner, inverse_of: :runner_namespaces, validate: true
     belongs_to :namespace, inverse_of: :runner_namespaces, class_name: '::Namespace'
     belongs_to :group, class_name: '::Group', foreign_key: :namespace_id
+
+    validates :runner_id, uniqueness: { scope: :namespace_id }
   end
 end