cron executes script more than once.

 Hello We have a cron job that handles bills and accounting...it is scheduled to run once per hour but yesterday the cron ran it twice which resulted with double billing our customer and it is not good. 

is there a way to determine why cron is executing script more than once? and how to configure cron to not execute the job more than once.. it does not matter if it not succeed as there is another cron running in a relatively short timeframe (60 minutes) 

the logs of /var/cron/log show that the scripts in the crontab are run more than once... why is that?