K-Rise Systems

Lets say I have a batch job that is scheduled to run every 1 minute.
the first time it runs, it takes 20 minutes.
at the end of 20 minutes, will it have run only once? Or at minute 2, will it see that the first is still running but it will start again anyway?

It will not start the batchjob again until the first one has finished. So in your example it will not start the job for a second time until minute 21.