If the scheduled start time of a job is passed - the job will be scheduled for the next scheduled start time. So if the job is scheduled to run at 12am and the current execution finishes at 00:00:00.003 then the job will be scheduled the following day.
There is no 'continuous' schedule available in SQL Server - if you need a job to execute continuously then you would create the code with an infinite loop so it executes the expected code and then repeats.
Note: for something like that I would definitely code a mechanism to stop the execution as well as code to disable the job. You might also consider a separate 'monitor' job that executes every xx minutes - which checks a table to determine if the job should be stopped, disabled, enabled, etc... and takes appropriate action (for example, if the job is supposed to be enabled and running - but is not - enabled the job and start it).
But - I would address the problem instead of the symptom...