Build failed with " You are already running a stack.sh session"

Jenkins failed a job on tempest checks

http://logs.openstack.org/20244/1/check/gate-tempest-devstack-vm/27762/console.html

15:22:45 [gate-tempest-devstack-vm] $ /bin/bash -xe /tmp/hudson5320904758380155333.sh
15:22:45 + export PYTHONUNBUFFERED=true
15:22:45 + PYTHONUNBUFFERED=true
15:22:45 + export DEVSTACK_GATE_TEMPEST=1
15:22:45 + DEVSTACK_GATE_TEMPEST=1
15:22:45 + cp devstack-gate/devstack-vm-gate-wrap.sh ./safe-devstack-vm-gate-wrap.sh
15:22:45 + ./safe-devstack-vm-gate-wrap.sh
15:23:28 Triggered by: https://review.openstack.org/20244 patchset 1
15:23:28 Pipeline: check
15:23:31 Running devstack
15:23:31 You are already running a stack.sh session.
15:23:31 To rejoin this session type 'screen -x stack'.
15:23:31 To destroy this session, type './unstack.sh'.
15:23:36 Build step 'Execute shell' marked build as failure