diff --git a/app/models/milestone.rb b/app/models/milestone.rb
index c06bfe0ccdd..b04bed4c014 100644
--- a/app/models/milestone.rb
+++ b/app/models/milestone.rb
@@ -107,7 +107,7 @@ class Milestone < ActiveRecord::Base
   end
 
   def participants
-    User.joins(assigned_issues: :milestone).where("milestones.id = ?", id)
+    User.joins(assigned_issues: :milestone).where("milestones.id = ?", id).uniq
   end
 
   def self.sort(method)
diff --git a/spec/models/milestone_spec.rb b/spec/models/milestone_spec.rb
index e3e8e6d571c..aa1ce89ffd7 100644
--- a/spec/models/milestone_spec.rb
+++ b/spec/models/milestone_spec.rb
@@ -249,4 +249,17 @@ describe Milestone, models: true do
       expect(milestone.to_reference(another_project)).to eq "sample-project%1"
     end
   end
+
+  describe '#participants' do
+    let(:project) { build(:empty_project, name: 'sample-project') }
+    let(:milestone) { build(:milestone, iid: 1, project: project) }
+
+    it 'returns participants without duplicates' do
+      user = create :user
+      create :issue, project: project, milestone: milestone, assignees: [user]
+      create :issue, project: project, milestone: milestone, assignees: [user]
+
+      expect(milestone.participants).to eq [user]
+    end
+  end
 end