3c993d78_SharePoint_Insight__Custom_Timer_Job
[0] Multiple Instances [PowerShell] No multiple instance guarantee. Administrator needs to make sure that no two instances are running at the same time. [Custom Timer Job] 
Custom Timer Jobs provides the power to specify Job LockTypes (i.e. SPJobLockTypes) which guarantees that multiple instances of same job will never execute at the same point in time.
SharePoint Insight
Custom Timer Job