At 14:46 Sussex EOC alerted the DVFD, Frankford VFC, SCEMS 103 & Medic 105 for reported structure fire. Engine 731 responded at 14:51 with a crew of (5) under the command of Rescue Capt. Hudson. Crews arrived to find a 2 1/2 story wood frame duplex with heavy fire on the #2 flr. & attic on side "C", with exposures to a 2 story barn. Rescue Capt. Hudson est. IC and requested 1 additional Engine Co. & a Truck Co. Engine 731 laid in from the hydrant & pulled a 1 3/4" line through the doorway of side "A". Next in Engine 763 pulled an additional attack line to side "C". Due to the interior stairwell being blocked by a commercial HVAC unit the crew from Engine 731 was forced to redeploy to side "C" as well. EOC alerted Millsboro for the Truck Co & Millville for the additional Engine Co. Engine/Tanker 73 & Ladder 83 arrived and were assigned to side "B" and roof and ordered to open up & exposure protection. Due to highs near 90 & high humidity additional Engine crews from Gumboro & Selbyville were transferred to the scene for manpower and overhaul. Millsboro & Bethany Beach provided the cover assignment for Dagsboro, while Roxana covered Frankford. IC placed the incident under control at 18:36 with crews working. The DSFMO determined the cause to be accidental in nature originating in the attic. Final crews cleared at 19:22. Great Job by all on the scene.