I think I got it. I just check for the boolean status of the timer before I start a new one AND make sure, there are not already other targets in the sensor.
Code:
def enter_sensor(e):
print('{0} entered {1}.'.format(e.target.name,e.sensor.name))
numTargets = len(e.manager.getActiveTargets(sensor=e.sensor))
global bTimerActive
if not bTimerActive and numTargets == 1:
print('Starting timer.')
viz.starttimer(0,concurrencyTolerance)
bTimerActive = True
But maybe there's a more elegant solution?