After upgrading from 1.8.15 to 1.9.1 (after the 1.9.0 failed yesterday) the Perforce plugin does not recognize existing builds.
When attempting a build it tries to create a new build number 1 (which already exists).
In this case it is a multibranch pipeline job (didn't test others).

In the log we see this:
Oct 02, 2018 9:11:47 AM jenkins.model.lazy.LazyBuildMixIn newBuild
WARNING: A new build could not be created in job ExternalLibraries/main
java.lang.IllegalStateException: JENKINS-23152: /var/lib/jenkins/jobs/ExternalLibraries/branches/main/builds/1 already existed; will not overwrite with ExternalLibraries/main #1
at hudson.model.RunMap.put(RunMap.java:189)
at jenkins.model.lazy.LazyBuildMixIn.newBuild(LazyBuildMixIn.java:185)
at jenkins.model.ParameterizedJobMixIn$ParameterizedJob.createExecutable(ParameterizedJobMixIn.java:511)
at jenkins.model.ParameterizedJobMixIn$ParameterizedJob.createExecutable(ParameterizedJobMixIn.java:321)
at hudson.model.Executor$1.call(Executor.java:365)
at hudson.model.Executor$1.call(Executor.java:347)
at hudson.model.Queue._withLock(Queue.java:1439)
at hudson.model.Queue.withLock(Queue.java:1300)
at hudson.model.Executor.run(Executor.java:347)

Oct 02, 2018 9:11:47 AM hudson.model.Executor run
SEVERE: Executor #-1 for master: Unexpected executor death
java.lang.Error: java.lang.IllegalStateException: JENKINS-23152: /var/lib/jenkins/jobs/ExternalLibraries/branches/main/builds/1 already existed; will not overwrite with ExternalLibraries/main #1
at jenkins.model.lazy.LazyBuildMixIn.newBuild(LazyBuildMixIn.java:193)
at jenkins.model.ParameterizedJobMixIn$ParameterizedJob.createExecutable(ParameterizedJobMixIn.java:511)
at jenkins.model.ParameterizedJobMixIn$ParameterizedJob.createExecutable(ParameterizedJobMixIn.java:321)
at hudson.model.Executor$1.call(Executor.java:365)
at hudson.model.Executor$1.call(Executor.java:347)
at hudson.model.Queue._withLock(Queue.java:1439)
at hudson.model.Queue.withLock(Queue.java:1300)
at hudson.model.Executor.run(Executor.java:347)
Caused by: java.lang.IllegalStateException: JENKINS-23152: /var/lib/jenkins/jobs/ExternalLibraries/branches/main/builds/1 already existed; will not overwrite with ExternalLibraries/main #1
at hudson.model.RunMap.put(RunMap.java:189)
at jenkins.model.lazy.LazyBuildMixIn.newBuild(LazyBuildMixIn.java:185)
... 7 more