I am also seeing this behavior. We are using jenkins for feature branch build with 8 workers. Whenever resources get free, up to 4 of the same branch will start building.
Same here.
Jenkins: 2.147
github plugin: 1.29.3
git client plugin: 2.7.3
git plugin: 3.9.1
Same here with 2.190.3. When push brings 2 new commits, 2 job starts. Both of them points to the Git repo's HEAD.