Add file RPC dup call

src/main.rs

@@ -73,6 +73,10 @@ impl file_rpc::Server for Serv {
         let (_mount_id, _file_offset, size) = self.files.read()[fd as usize];
         Some(size as u64)
     }
+
+    fn dup(&self, fd: u64) -> Option<u64> {
+        Some(fd)
+    }
 }
 
 fn main() {