Comment activity creates a new activity entry on every comment. It currently doesn't have any behavior for grouping those together. So what you're seeing is normal for now. Will likely add a delete action to the comment activity action to purge the previous on new. You can do this now if you like to see if it works by making a copy of the existing comment activity action then adding the below before the new activity create.
Mode: Activity
Method: Delete
Asset: [var3_asset].comment
Load: By Asset Only
Another option is in the copy change "Load" to "By Asset Only" instead of deleting the previous it simply updates it with a new owner. Regardless of either of these the original entry will still always display. Have added a feature ticket to adjust the load parameter as a solution for now.
forge.joomlapolis.com/issues/6611