For the duplexer cost, until now I found 2 offers :
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The first one was recommended by @cswiger in this post : OAI Progress - next error. It’s cheap but they didn’t replied to my e-mails regarding checkout and shipping method.
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Duplexer made of cavity filters I think much better but more expensive : https://open-cells.com/index.php/opencellsband7duplexer/