A change introduced since the release of git plugin 2.3.5 has caused the plugin to start a new build each time the git server is polled for changes. It does not do that for every job, and I'm not clear why it does it for this specific job definition, but I've confirmed several times that this configuration has the problem, and that it did not have the problem with 2.3.5 and earlier.

Steps to duplicate the problem:

1. Clone a docker repo with the base config on a branch

   git clone https://github.com/MarkEWaite/docker.git
   

2. checkout the docker branch to show the problem

   cd docker
   git checkout -b master-JENKINS-26179-poll-now-always-starts-a-build -t origin/master-JENKINS-26179-poll-now-always-starts-a-build
   

3. Build the docker image

   sudo docker build -t markewaite/JENKINS-26179 .
   

4. Run the docker image

   sudo docker run -p 9090:8080 -p 50000:50000 -i -t markewaite/JENKINS-26179
   

5. Create the job

   java -jar jenkins-cli-jar -s http://localhost:9090 create-job JENKINS-26179 < JENKINS-26179.xml
   

6. Poll for changes so the job will build - Confirm that it builds 5 times (once for each branch)
7. Press "Poll Now" - Confirm that (as expected) it does not build again
8. Install git plugin pre-release 2.3.6
9. Restart Jenkins (to activate the enw git plugin version)
10. Press "Poll Now" - Confirm that (unexpected - the bug) it builds each time "Poll Now" is pressed