Briggs and stratton model 21b907-0029 it's a new engine with less than 5 hours on it . When I run the engine and then shut it down for about 5 minutes , when I go to restart it ,it backfires once and then I start it and it runs fine . Is this normal and will it hurt anything

Hi,
I would not call it normal, but it is indicative of the engine in certain appplications. It is not the engine that causes it, but the muffler attached to it. Many mufflers can be installed to an engine depending on the cost, noise level, application, etc. Of those cost is the usual factor that "wins" based on the total equipment cost.
Peace,
Clarke