Update to new struct version of FileWriteMode

src/main.rs

@@ -91,8 +91,7 @@ impl file_rpc::Server for Serv {
     }
 
     fn dup(&self, fd: u64) -> Result<u64, Errno> {
-        let file = self.open_files.get(fd as usize).ok_or(Errno::EBADF)?.clone();
-        self.open_files.insert(file).map(|x| x as u64).ok_or(Errno::ENOMEM)
+        Ok(fd)
     }
 
     fn seek(&self, fd: u64, pos: file_rpc::SeekFrom) -> Result<u64, Errno> {