Update to new struct version of FileWriteMode

src/main.rs

@@ -41,10 +41,7 @@ impl file_rpc::Server for Serv {
     }
 
     fn dup(&self, fd: u64) -> Result<u64, Errno> {
-        let mut files = self.files.write();
-        let file = files[fd as usize];
-        files.push(file);
-        Ok((files.len() - 1) as u64)
+        Ok(fd)
     }
 
     fn seek(&self, fd: u64, pos: file_rpc::SeekFrom) -> Result<u64, Errno> {