Despite your best efforts, it's still possible to end up with duplicate hours for individual volunteers. Sometimes those duplicate hours are intentional, for example your volunteers enter hours quarterly and all of Q3's hours are entered with a start date of July 1. And sometimes they're not...
From the Administrator tools panel, click on Find overlapping volunteer hours.
Each record here is an hours record that, for the same volunteer, overlaps one or more other hours records. Both overlapping hours for the same person show here. If all are okay, leave them un-changed, otherwise edit or delete one of the records.