How to run a cron job each time the previous execution has finished?
In terminal:

while :; do ./script_name; done


or

while :; do python programm.py; done


If you want add some sleep time before run program again use 'sleep' command:

For example:

while :; do python programm.py; sleep 90; done


in this example cycle will sleep 90 second before run again you program