bug: A user is able to send multiple relation requests to the same member

**Describe the bug**
A user is able to send multiple relation requests to the same member. Ideally, if a request has already been sent to a member, the backend should return that as a message for a duplicate request to the same member.

**To Reproduce**
Steps to reproduce the behavior:

1. Use send request API on the same user multiple times
2. See all of them succeeding (and also in sent relation requests)

**Expected behavior**
Ideally, if a request has already been sent to a member, the backend should return that as a message for a duplicate request to the same member.

**Screenshots**
Received multiple requests from the same user. Accepted one of them, and then cancelled. Old requests still stay. The complete UX flow can probably be fixed by doing a check for already sent requests and not allowing duplicate requests.

**Smartphone (please complete the following information):**
- Device: iPhone
- OS: iOS 13.6
- Discovered using Mentorship iOS app