overlap in job

It is written that next run date is calculated just before execution of job. If this is case then if a job takes 10 minutes to execute and job interval is 2 minutes then job overlapping is possible ? But I think because of JQ lock overlapping is not possible. To prevent job overlapping , next run date needs to be calculated at the end. So which one is correct ? IF overlapping does not happen and next run date is also calculated at begining then how the above mentioned case will be handled ?

Deba