If you're submitting into the same form multiple times within short period and entered the same data, it won't work indeed. MachForm will consider it as duplicate entry and simply discard it (hence the redirect won't work).
If you don't want the duplicate-submission filter being triggered, you can turn it off. Edit your config.php file, search for this:
define('MF_DISCARD_DUPLICATE_ENTRY',true);
change it to become:
define('MF_DISCARD_DUPLICATE_ENTRY',false);
Restart your browser before testing it again and it should be working fine now.