We have been getting this error for the past day or so.
Per instructions at https://wiki.jenkins-ci.org/display/JENKINS/Dead+Executor, am filing a JIRA bug.

May 04, 2016 11:49:24 AM hudson.WebAppMain$3 run
INFO: Jenkins is fully up and running
May 04, 2016 11:49:33 AM javax.jmdns.impl.DNSStatefulObject$DefaultImplementation waitForAnnounced
WARNING: Wait for announced timed out: DNS: JSERVER07.local. state: announcing 1 task: Announcer(JSERVER07.local.) state: announcing 1
May 04, 2016 11:49:39 AM javax.jmdns.impl.DNSStatefulObject$DefaultImplementation waitForAnnounced
WARNING: Wait for announced timed out: DNS: JSERVER07.local. state: probing 1 task: Prober(JSERVER07.local.) state: probing 1
May 04, 2016 11:49:45 AM javax.jmdns.impl.DNSStatefulObject$DefaultImplementation waitForAnnounced
WARNING: Wait for announced timed out: DNS: JSERVER07.local. state: probing 1 task: Prober(JSERVER07.local.) state: probing 1
May 04, 2016 11:51:07 AM hudson.model.Executor run
SEVERE: Unexpected executor death
java.lang.IllegalStateException: /var/lib/jenkins/builds/my_project/2 already existed; will not overwrite with my_project #2
at hudson.model.RunMap.put(RunMap.java:188)
at jenkins.model.lazy.LazyBuildMixIn.newBuild(LazyBuildMixIn.java:176)
at hudson.model.AbstractProject.newBuild(AbstractProject.java:1019)
at hudson.model.AbstractProject.createExecutable(AbstractProject.java:1210)
at hudson.model.AbstractProject.createExecutable(AbstractProject.java:145)
at hudson.model.Executor$1.call(Executor.java:364)
at hudson.model.Executor$1.call(Executor.java:346)
at hudson.model.Queue._withLock(Queue.java:1405)
at hudson.model.Queue.withLock(Queue.java:1270)
at hudson.model.Executor.run(Executor.java:346)