Since I’ve added this friend, they constantly share non-existent matches with me, which are then all marked as duplicates. I’ve asked her and she says she doesn’t, and that she experiences the same thing with a different friend. All those matches are shared by her, they are all marked as skipped because duplicates, and they are not limited to times where either of us is playing/uploading.
Hey, that’s a weird one. It should now be sorted - it seems that if they shared a match you already had, the sharing task would fail (correctly, as you’ve already got the match) but not mark itself as “complete” so it would keep retrying again later and well, spam notifications (at least it somewhat groups them I suppose… 
). I’m honestly amazed this hasn’t been reported before now.
There shouldn’t be any impact beyond the notification spam - please let me know if it still occurs & thanks for the report.