Case Summary displays duplicate entries for some activity records

We updated this site to 4.7.15 in late January. Since that time we are seeing that Case Summaries (which show a listing of all activities included in the case) are displaying two identical rows for each activity (including identical URLs), but not for every activity type. Activity records in the case that were created as part of the standard timeline do not appear to be displayed twice, where subsequently added activities are listed twice.