fix(events): prevent duplicate participation in participateEvent

[anshul23102]

Related Issue

Closes #278

Problem

participateEvent in eventService.js pushed a commit entry unconditionally:

event.commits.push({ user: userId });
await event.save();

Calling the endpoint multiple times with the same userId and eventId created duplicate entries in event.commits, inflating the displayed participant count for organisers.

Fix

Added an idempotency guard before the push:

const alreadyParticipated = event.commits.some(
  (commit) => commit.user.toString() === userId.toString()
);

if (alreadyParticipated) {
  const error = new Error('User has already participated in this event');
  error.statusCode = 409;
  throw error;
}

The thrown error carries statusCode = 409. The global error handler in error.middleware.js reads err.statusCode (line 22) and forwards it as the HTTP status, so the endpoint returns 409 Conflict on duplicate attempts without any changes to the middleware.

Files Changed

File	Change
backend/services/eventService.js	Add participation duplicate check before event.commits.push

Testing

• First RSVP for a given user and event: HTTP 200.
• Second RSVP with the same user and event: HTTP 409 Conflict.
• Different user RSVPing for the same event: HTTP 200.

---

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