{!--example url: template_group/template/123--}
{exp:safecracker entry_id="{segment_3}" return="template_group/template/ENTRY_ID"}

...

{!--
this is the trick that duplicates the entry
even though entry_id 123 is LOADED into this form
this blank entry_id field will trick SC into thinking
this is a new entry
--}

<input type="hidden" name="entry_id" value="">

{/exp:safecracker}

{!-- add this code if you are using Matrix (w/ PHP on Input) --}
<?php $_GET['clone'] = 'y' ?>