Builds configured with manual stages do not prevent concurrent builds

If a build is 'paused' waiting for a manual stage to be started, this state does not prevent a new instance of this build from starting (even with build concurrency disabled).

Due to this if you have a Parent build, which then triggers the Child build (and the Child has the manual stage). There will be several instances of the Child build having been triggered, and waiting for the manual stage to be fired.

There should be an option to prevent additional triggering of a build while waiting for the manual stage to be started.