Investigate & fix how someone was able to buy the membership twice (via stripe and pretix)

[marcoacierno]

Someone was able to pay for a membership twice via stripe subscriptions and Pretix

Our system detected it and didn't store the duplicate payment (see Slack #website-alerts).

We should investigate:

1. How was it possible? PyCon should show the message "You are already a member"
2. Should we store the duplicate payments anyway? so we can keep track of each payment. Right now the duplicate payment was not stored, so we wouldn't know about this without the error

association=> select * from pretix_payments where payment = 65;
 id | payment | order_code | event_organizer | event_id
----+---------+------------+-----------------+----------
(0 rows)

For (1) I wonder if it is caused by the button showing "Add membership" if the user is not logged in.