Ensure group_users to return distinct users

Caused uniqueness constraint when inserting as users

ee/app/models/concerns/approval_rule_like.rb

@@ -8,7 +8,7 @@ module ApprovalRuleLike
   included do
     has_and_belongs_to_many :users
     has_and_belongs_to_many :groups, class_name: 'Group', join_table: "#{self.table_name}_groups"
-    has_many :group_users, through: :groups, source: :users
+    has_many :group_users, -> { distinct }, through: :groups, source: :users
 
     validates :name, presence: true
   end

ee/spec/models/concerns/approval_rule_like_spec.rb

@@ -87,4 +87,16 @@ describe ApprovalRuleLike do
 
     it_behaves_like 'approval rule like'
   end
+
+  context '.group_users' do
+    subject { create(:approval_project_rule) }
+
+    it 'returns distinct users' do
+      group1.add_guest(user1)
+      group2.add_guest(user1)
+      subject.groups = [group1, group2]
+
+      expect(subject.group_users).to eq([user1])
+    end
+  end
 end