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I found a pair of places where the attendee count was being miscalculated.
This only occurs if something posts to the API to make the user attend an event, and the thing making the calls hasn't pre checked the user attend status, and the API doesn't currently check.
So my user_attend table has a fair few rows where the EID and UID is the same.
On event row view (so /event/)
In the attendee list on the event details page when you select the show button.
(The count on the event view page is correct)
I've already fixed and GitHub pull requested, then I read the contribution guide. Hence doing things in the wrong order.
GitHub pull request 758 (https://github.com/joindin/joind.in/pull/758)