Is there a reason to install anti-siphon valves and a backflow protector on the main line?
I agree it seems redundant to use anti-siphon valves in combination with a backflow prevention device. But people don't always choose these valves because they needed the anti-siphon feature. Maybe the person who chose them just doesn't like operating or maintaining valves that are underground (harder to bend and reach) in a box full of spider webs. They could have preferred the up-and-down arrangement of the above ground anti-siphon type rather than the basic inline type.
Your &quot;authority having jurisdiction&quot; gets to have the final word on the matter, however. Call your water supplier and ask them whether you have an accepted type of backflow prevention device and whether they require anti-siphon valves too.