(As a side note: Thank you for this great plugin!)

After an upgrade from version 0.8.9 to 0.9.1, the following warnings started to appear in the logs, every time a delivery pipeline view is accessed:

May 28, 2015 11:45:59 AM se.diabol.jenkins.pipeline.util.ProjectUtil getAllDownstreamProjects
WARNING: Project Integration branches » selenium_tests already exists as a downstream project.
May 28, 2015 11:45:59 AM se.diabol.jenkins.pipeline.util.ProjectUtil getAllDownstreamProjects
WARNING: Project Integration branches » selenium_tests already exists as a downstream project.

Our setup includes a project with a join trigger (from the join plugin):
Project A runs downstreams projects B, C and then uses the Join Trigger as a post-build action to run project D (the "selenium_tests" project from the warning messages above).
Project D appears as downstream project for projects B and C in the user interface.

It appears that project D is seen multiple times as a downstream job (via the paths A-B-D and A-C-D). In this case a warning is not warranted as this is a valid configuration.