problems with engine turning over a little longer before actually starting

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This is happening sporadically. Usually after a short trip, less than five miles. When I park and run a short errand, and come back to start again, it acts like it may not ignite engine. Turning over, but not starting for a couple of extra seconds. Then it does start up. Had it happen today again and it sputtered a little as it was turning over before starting. Took it to dealer but they could not reproduce the problem.