For some reason (not documented in the POM) blue ocean uses a the non preffered way of invoking jacoco for code coverage.

http://www.eclemma.org/jacoco/trunk/doc/instrument-mojo.html

The issue with this is that classess are not restored until prepare-package which is not called when running mvn test

Thus when you run mvn test followed by mvn test you end up with a failure as jacoco tries to instrument already instrumented classes and bombs out. Workaround it to run mvn clean test but this adds to the turnaround time to check tests.

Note you can just enable jacoco with mvn -Penable-jacoco with recent jenkins plugin poms and it does the right thing...

ERROR] Failed to execute goal org.jacoco:jacoco-maven-plugin:0.7.9:instrument (default-instrument) on project blueocean-git-pipeline: Unable to instrument file.: Error while instrumenting class /mnt/c/workarea/source/github/jenkinsci/blueocean-plugin/blueocean-git-pipeline/target/classes/io/jenkins/blueocean/blueocean_git_pipeline/GitPipelineCreateRequest.class. Class io/jenkins/blueocean/blueocean_git_pipeline/GitPipelineCreateRequest is already instrumented. -> [Help 1]