Resource and Job Workbench in EBS R12

Morning,

we recently started to explore using the resource and job workbench functions, but seem to have run into an issue that should not come up.  On both workbenches, you are allowed to move a resource back to a time when the previous resource has not even completed the task yet. To me, this cannot occur logically.  Is there a set up or a profile option that needs to be turned on/off or is there a bug possible?

Or is there a better tool that deals with resource capacity

any feedback would be helpful