If I have a crontab job that runs, for instance, every hour, but that job may take more than an hour to complete, will the next job fail to run?
Can a crontab job run concurrently with itself?
cron
No, cron scripts run parallelly, if you do not implement somelocking mechanism.See Quick-and-dirty way to ensure only one instance of a shell script is running at a time  and  Correct locking in shell scripts? for possible solutions.A simpler  way is to use lockfile, like in this answeror the run-one package (see this answer)- thanks to gertvdijk for suggesting it.