On December 17, 2006, at 1115 mountain standard time, a Piper PA 23-235, N5829X, had the landing gear collapse at Phoenix Deer Valley Airport, Phoenix, Arizona. The commercial pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The pilot, who was also the registered owner of the airplane, was operating it under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed and no flight plan was filed. The flight was originating at the time of the accident. In a telephone conversation immediately following the accident, the pilot stated that the airplane was positioned in the run up area for runway 25L and he had completed the run up. As the engines were powering down, the right main gear collapsed. In a written statement, the pilot reported that prior to starting the airplane's engines, he lowered the gear handle and raised the flap handle. Once the left engine was started, the gear handle and flap handle went to neutral and the pilot verified that he had three green landing gear lights, indicating that the gear was down and locked. The pilot taxied the airplane to the run up area for runway 25L. At the run up area, he proceeded through his checklist, and ran the engine checks at 2,200 revolutions per minute. Then, the pilot idled the engines and completed the checklist. He released the brakes to taxi to the runway for takeoff, and the right main gear collapsed. The pilot noted that the right wing impacted the ground, the right rear window was broken, and the fuselage skin was wrinkled behind the right rear window. According to the pilot, the airplane was involved in a gear up landing previously and had undergone repairs. Since the repairs had been completed, the pilot did not note any problems with the landing gear system. The maintenance manual for the airplane indicates that, "...the tricycle landing gear system is an air-oil oleo type unit that is hydraulically operated and fully retractable with the nose gear retracting aft into the nose section and the main gear retracting forward into the engine nacelles. To prevent the gear from being retracted while the airplane is on the ground, an anti-retraction valve, located on the left main gear, prevents a build up of hydraulic pressure in the retraction system while the weight of the airplane is on its wheels." According to the Pilot's Operating Handbook for the airplane, after the left engine is started, the landing gear handle should be lowered and then the pilot should verify that it returns to a neutral position. The airplane manufacturer's representative indicated that although the pilot lowered the landing gear handle prior to starting the left engine, the action should have had no adverse effect on the operation of the landing gear system. The representative further stated that the anti-retraction valve works to stop on- ground gear retraction at low revolutions per minute. If power is applied to move the airplane, or if a run-up is performed, the possibility exists for the gear to unlock if the landing gear selector is in the retracted position. Aircraft recovery personnel reported that during the recovery of the aircraft, none of the landing gear were in the down-and-locked position. The airplane was lifted using a crane and the landing gear was cycled to the down-and-locked position. The wings were removed from the aircraft and no further functional testing was performed. Examination of the landing gear system linkage and anti-retraction valve did not reveal any mechanical anomalies.