Fix listing of epics by milestone

When epics are filtered by milestone, this is done through `join` issues
table. If an epic has multiple issues from the same milestone, then
there are duplicit Epic instances returned - we should use distinct list
in this case.

ee/app/models/ee/epic.rb

@@ -71,7 +71,7 @@ module EE
       scope :in_parents, -> (parent_ids) { where(parent_id: parent_ids) }
       scope :inc_group, -> { includes(:group) }
       scope :in_selected_groups, -> (groups) { where(group_id: groups) }
-      scope :in_milestone, -> (milestone_id) { joins(:issues).where(issues: { milestone_id: milestone_id }) }
+      scope :in_milestone, -> (milestone_id) { joins(:issues).where(issues: { milestone_id: milestone_id }).distinct }
       scope :in_issues, -> (issues) { joins(:epic_issues).where(epic_issues: { issue_id: issues }).distinct }
       scope :has_parent, -> { where.not(parent_id: nil) }
       scope :iid_starts_with, -> (query) { where("CAST(iid AS VARCHAR) LIKE ?", "#{sanitize_sql_like(query)}%") }

ee/changelogs/unreleased/fix_duplicit_epics_on_roadmap.yml

+---
+title: Fix duplicit epics on roadmap page when filtering by milestone
+merge_request: 52201
+author:
+type: fixed

ee/spec/finders/epics_finder_spec.rb

@@ -398,7 +398,7 @@ RSpec.describe EpicsFinder do
 
                   params[:milestone_title] = ancestor_group_milestone.title
 
-                  expect(epics(params)).to contain_exactly(ancestor_epic1, ancestor_epic1, subgroup_epic1)
+                  expect(epics(params)).to contain_exactly(ancestor_epic1, subgroup_epic1)
                 end
 
                 it_behaves_like 'filtered by milestone', :project do

ee/spec/models/epic_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Epic do
 
   let_it_be(:user) { create(:user) }
   let_it_be(:group) { create(:group) }
-  let(:project) { create(:project, group: group) }
+  let_it_be(:project) { create(:project, group: group) }
 
   describe 'associations' do
     subject { build(:epic) }
@@ -61,6 +61,21 @@ RSpec.describe Epic do
         expect(described_class.order_relative_position_on_board(board.id)).to eq([epic2, epic3, epic1, public_epic, confidential_epic])
       end
     end
+
+    describe '.in_milestone' do
+      let_it_be(:milestone) { create(:milestone, project: project) }
+
+      it 'returns epics which have an issue in the milestone' do
+        issue1 = create(:issue, project: project, milestone: milestone)
+        issue2 = create(:issue, project: project, milestone: milestone)
+        other_issue = create(:issue, project: project)
+        epic1 = create(:epic_issue, issue: issue1).epic
+        epic2 = create(:epic_issue, issue: issue2).epic
+        create(:epic_issue, issue: other_issue)
+
+        expect(described_class.in_milestone(milestone.id)).to match_array([epic1, epic2])
+      end
+    end
   end
 
   describe 'validations' do