User can join club repeatedly through invites.

[randysim]

If a leader of a club gets invited to join the club on stuyactivities, they can click that invite link over and over again generating an inflated member count. To fix this, just check if the user is already in the club when adding the user.

[TheEgghead27]

Note that there may be race conditions, especially in adding membership requests, due to database desync.

[TheEgghead27]

In addition, when a bug fix is deployed, we should take care to scan for and purge extraneous memberships/requests

[TheEgghead27]

Notes:

• GraphQL API route rate-limiting for join requests and membership approvals should mitigate the race conditions.
• Invitations must be checked for valid users
• SQL may be used at the resolution of this bug to purge existing duplicate memberships/invitations