Fix pagination on sorts with lots of ties

Postgres and MySQL don't guarantee that pagination with `LIMIT` and
`OFFSET` will work if the ordering isn't unique. From the Postgres docs:

> When using `LIMIT`, it is important to use an `ORDER BY` clause that
> constrains the result rows into a unique order. Otherwise you will get
> an unpredictable subset of the query's rows

Before:

    [1] pry(main)> issues = 1.upto(Issue.count).map { |i| Issue.sort('priority').page(i).per(1).map(&:id) }.flatten
    [2] pry(main)> issues.count
    => 81
    [3] pry(main)> issues.uniq.count
    => 42

After:

    [1] pry(main)> issues = 1.upto(Issue.count).map { |i| Issue.sort('priority').page(i).per(1).map(&:id) }.flatten
    [2] pry(main)> issues.count
    => 81
    [3] pry(main)> issues.uniq.count
    => 81

1 files changed, 14 insertions, 0 deletions

diff --git a/spec/models/concerns/issuable_spec.rb b/spec/models/concerns/issuable_spec.rb
index efbcbf72f76..89730ab8eb8 100644
--- a/spec/models/concerns/issuable_spec.rb
+++ b/spec/models/concerns/issuable_spec.rb
@@ -154,6 +154,20 @@ describe Issue, "Issuable" do
         expect(issues).to match_array([issue1, issue2, issue, issue3])
       end
     end
+
+    context 'when all of the results are level on the sort key' do
+      let!(:issues) do
+        10.times { create(:issue, project: project) }
+      end
+
+      it 'has no duplicates across pages' do
+        sorted_issue_ids = 1.upto(10).map do |i|
+          project.issues.sort('milestone_due_desc').page(i).per(1).first.id
+        end
+
+        expect(sorted_issue_ids).to eq(sorted_issue_ids.uniq)
+      end
+    end
   end