Next slot also unavailable when duration and interval are same

When interval and duration are same and are set in minutes, after booking one time slot, it also makes next one unavailable.

Example: Duration: 60min Interval: 60 min

When booked let say Feb, Thursday 12, 2017 at 9:00 AM, the slot Feb, Thursday 12, 2017 at 10:00 AM become also unavailable. If duration is set to 59 min, everything works like it should.