Duplicate Activity

For the main Activity stream if someone comments it duplicates the activity display. So the original post is displayed, and so is the one with the comment.
On the Group Activity stream the activity only appears once?!?
It didn't used to do this. I remember a setting that prevented duplicates. How do I switch off duplicates appearing?

J: 5.4.0
CB: 2.11.0

You can adjust what activity is logged within CB Activity > Parameters > Activity. Sounds like you have commenting activity enabled for something. Simply disable it.

These are activity reactions and not necessarily duplicates. User A makes Post X and User B comments on Post X making a "commented on Post X" activity entry.

Ahhh the redundant activity log is from "media" - so if I load a photo it makes a second redundant post. 
But I don't see "media" in CB Activity > Parameters > Activity and it's not Gallery - because I switched all Gallery logging off.

That's a bug already fixed in latest CB Gallery and CB Activity build releases. If you update both to latest that should be fixed. The views tracking was incorrectly triggering the media update event when it shouldn't have.