Streamline icon use for consistent UI

Replacing instances of 'issue-duplicate' with 'duplicate'
to use one icon for the same task. This also will allow
us to keep the icon library efficient.

app/helpers/system_note_helper.rb

@@ -23,7 +23,7 @@ module SystemNoteHelper
     'moved' => 'arrow-right',
     'outdated' => 'pencil-square',
     'pinned_embed' => 'thumbtack',
-    'duplicate' => 'issue-duplicate',
+    'duplicate' => 'duplicate',
     'locked' => 'lock',
     'unlocked' => 'lock-open',
     'due_date' => 'calendar'

changelogs/unreleased/208798-replace-instances-of-the-issue-duplicate-icon-with-the-duplicate-i.yml

+---
+title: Replace issue-duplicate icon with duplicate icon
+merge_request:
+author:
+type: other