Every message in agent log appears twice with the -agentLog option

Running a local slave, launched using a command, with the -slaveLog option causes a log file to be written (as expected) with every message printed twice there, like this:

channel startedchannel started

channel stoppedchannel stopped

(which is not expected).

The launch script looks as follows (/home/matej/wc/jenkins is path to the Jenkins clone):

#!/bin/bash --norc
export TMP=$(mktemp -d --tmpdir jenkins-XXXXX)
SLAVEJAR_URL=http://localhost:8080/jenkins/jnlpJars/slave.jar
wget -q "${SLAVEJAR_URL:?}" -P "${TMP:?}" 2>&1
_log_dir=/home/matej/wc/jenkins/work/logs
mkdir -p "$_log_dir"
_log=$(mktemp -u "$_log_dir/$(date -u +%Y%m%d-%H%M%S)-XXX")
export JAVA_HOME=/usr/lib/jvm/java-7-oracle
"$JAVA_HOME"/bin/java -jar "$TMP"/slave.jar \
                      -slaveLog "$_log"

I wondered whether it is just the messages that gets duplicated (perhaps caused by the TeeOutputStream created by the Launcher effectively outputting twice to the identical output stream) or if it is the threads calling Launcher.run() (I cannot imagine how that could happen).