Project should not be queued in the build agent while it is currently building there

I have a project with 2 build definitions and both have the same build environment( with attached build agent A)

1. Build project with build definition A
2. Project will build in build agent A
3. while project is still building, build project again using build definition B

Actual Result: Project will be queued/build in build agent A

Expected Result: Project will not be queued/build in build agent A because it's still building there.

If the build agent group only consist of one build agent, then the project will not build at all.
If the build agent group contains many build agent, then the project will build on the other build agents in the group where it isn't currently building.