Hi. This case is very similar to this one.

1) userA uploaded photo to group's timeline
2) I commented it
3) userB commented it too
4) userC commented it too...
5) userD commented it too...
Result: I get 3+ separated notification about "User* also commented UserA's photo".

I suppose:
1) system it should combine all notifications to one notification and get SINGULAR and PLURAL versions (as for other activity).
2) only one comment is publiched, system should display comment text (quote).

Example stream item: https://crimson-land.ru/community/stream/item/57143

few or one of this:
APP_GROUP_PHOTOS_NOTIFICATIONS_COMMENTED_USERS_PHOTO_SHARED_ON_THE_STREAM="%1$s also posted a comment on the photos shared by {b}%2$s{/b}"
APP_GROUP_STORY_USER_COMMENTED_ON_USERS_SHARED_PHOTO="%1$s also commented on the photo {b}%2$s{/b} shared in {b}%3$s{/b}"
APP_USER_PHOTOS_NOTIFICATIONS_COMMENTED_USERS_PHOTO_SHARED_ON_THE_STREAM="%1$s also posted a comment on the photos shared by {b}%2$s{/b}"

Thanks.