[Synthetics] Monitor add route allows duplicate monitor names

[awahab07]

Kibana version: 8.12.0

Elasticsearch version:

Server OS version:

Browser version:

Browser OS version:

Original install method (e.g. download page, yum, from source, etc.):
Tested on 8.13.0-SNAPSHOT

Describe the bug:
Although the Synthetics UI validates the monitor for duplicate names but the server route does not. So if there's some re-trying due to how the browser or the network behaves, it may recreate many duplicate monitors.

Steps to reproduce:

1. Go "Create monitor" page on Synthetics UI and create an HTTP monitor with simplest configuration.
2. Before pressing save, open browser Network panel in dev tools to track the POST request to /internal/synthetics/service/monitors route.
3. Hit save and "Copy as fetch" the sent request.
4. Issue the same request from browser console a few time.
5. Note that the requests go successful resulting in many duplicate monitors.

Expected behavior:
The route should return Baa Request with error message that a monitor with the same name has already been created.

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:
The issue has been reported on forum.

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[dominiqueclarke]

Resolved by 29e8394