you do not need crontab entries to run scripts unless you just want them ... the user in question may be running the script directly on the command line or using at so there is no need to modify crontab ... also, the issue may be that it is running in a user's crontab so you may want to check the user crontab files for the entry ... i think your suspect is very likely running the script manually based on your description ...

if you need to restrict the script to one process only while it is running, modify your shell script to use a marker file or at least check for an existing PID of the process prior to continuing the script ... a search of the forums or google should show you how to do so ...