Extractor fan back draught shutter - one flap or two?

Having felt a cold incoming draught from the bathroom extractor fan I'm looking at adding a back draught shutter to the pipe run.

As usual, nothing is easy. I can have one flap or two.

Advantages of one over the other?

Cheers

Dave R

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David
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noisy things don't bother

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Jim GM4DHJ ...

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