reverted uniqueness constraint on mime_type errors due to bug in notifications uniqueness filtering

indra/newview/skins/default/xui/en/notifications.xml

@@ -4629,9 +4629,6 @@ If you want to view streaming media on parcels that support it you should go to
    name="NoPlugin"
    type="notify">
 No Media Plugin was found to handle the "[MIME_TYPE]" mime type.  Media of this type will be unavailable.
-  <unique>
-    <context key="MIME_TYPE" />
-  </unique>
   </notification>
   <notification
    icon="alertmodal.tga"