Add flag to disable warning logs for duplicate gossip messages (#5009)

* Add flag to disable warning logs for duplicate gossip messages.

* Update Lighthouse book.

beacon_node/src/config.rs

@@ -1395,6 +1395,9 @@ pub fn set_network_config(
             Some(config_str.parse()?)
         }
     };
+
+    config.disable_duplicate_warn_logs = cli_args.is_present("disable-duplicate-warn-logs");
+
     Ok(())
 }