Sometimes a cron job might run slower, than you anticipated and many instances of it stack up.
You can increase the time interval during starts of the cron job - this can lead to problems in the future however.
Fortunately there's a easy and clean fix.
Just replace your cronjob with this:
/usr/bin/flock -n /var/lock/<foobar> <long-running-task>
Whenever your cron job is started a lock file will be created and no other instance can start.
This a simple remedy for colliding cron job's. I use it frequently.