diff --git a/db/migrate/20180508102840_add_unique_constraint_to_project_mirror_data_project_id_index.rb b/db/migrate/20180508102840_add_unique_constraint_to_project_mirror_data_project_id_index.rb
new file mode 100644
index 0000000000000000000000000000000000000000..acb976b52fa3b7dbdeb1636758fe2ca40712287e
--- /dev/null
+++ b/db/migrate/20180508102840_add_unique_constraint_to_project_mirror_data_project_id_index.rb
@@ -0,0 +1,31 @@
+class AddUniqueConstraintToProjectMirrorDataProjectIdIndex < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:project_mirror_data,
+                         :project_id,
+                         unique: true,
+                         name: 'index_project_mirror_data_on_project_id_unique')
+
+    remove_concurrent_index_by_name(:project_mirror_data, 'index_project_mirror_data_on_project_id')
+
+    rename_index(:project_mirror_data,
+                 'index_project_mirror_data_on_project_id_unique',
+                 'index_project_mirror_data_on_project_id')
+  end
+
+  def down
+    rename_index(:project_mirror_data,
+                 'index_project_mirror_data_on_project_id',
+                 'index_project_mirror_data_on_project_id_old')
+
+    add_concurrent_index(:project_mirror_data, :project_id)
+
+    remove_concurrent_index_by_name(:project_mirror_data,
+                                    'index_project_mirror_data_on_project_id_old')
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 6f40f70d350471b5303f4d18cc46fb8c0defd95f..ec0aa5f6603411ac25415624c5cd3d22e5a19a07 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180508100222) do
+ActiveRecord::Schema.define(version: 20180508102840) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1536,7 +1536,7 @@ ActiveRecord::Schema.define(version: 20180508100222) do
   end
 
   add_index "project_mirror_data", ["jid"], name: "index_project_mirror_data_on_jid", using: :btree
-  add_index "project_mirror_data", ["project_id"], name: "index_project_mirror_data_on_project_id", using: :btree
+  add_index "project_mirror_data", ["project_id"], name: "index_project_mirror_data_on_project_id", unique: true, using: :btree
   add_index "project_mirror_data", ["status"], name: "index_project_mirror_data_on_status", using: :btree
 
   create_table "project_statistics", force: :cascade do |t|