A warning should precede a time-sheet submission where a previous time sheet is unsubmitted. On the occurrence of a new week a new time-sheet becomes available making it easy to mistake this week for the prior week; especially, when one doesn't usually look at the week.
We do have an option to send out notifications for the overdue timesheets. Are you already using that for the employees?
We can also explore the option of customizing the validation rules for you. This will give us an opportunity to check if we can customize and provide you with this capabilities. Would you like one of our Support representatives to reach out to you regarding this?
Charlene, I believe that would work well. The problem lies in users inadvertently picking up a new timesheet. This would make that unlikely. (We do have notices for overdue timesheets - this is exactly when the problem occurs)