Many two-inlet (2 x 2-1/2") fire department connections I'm familiar with and commonly have a single-clapper, where the clapper will swing and cover the inside of the inlet on the side of the FDC not being used to prevent flow out of the other inlet.
Is there any advantage to having a double clapper?
One disadvantage I see is that you can't forward-flow test the backflow preventer out the FDC if there are clappers for each inlet.
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