I wanted to copy the extra fields also if I'm copying a products to another place. I found this in my old files,it's working for me:
To Copy extra fields
Look for (in catalog/admin/categories.php):
$dup_products_id = tep_db_insert_id();
Add after it:
// BOF: Extra fields to database on duplicate
$xf_query = tep_db_query("select products_id, products_extra_fields_id, products_extra_fields_value from " . TABLE_PRODUCTS_TO_PRODUCTS_EXTRA_FIELDS . " where products_id = '" . (int)$products_id . "'");
while($xfield = tep_db_fetch_array($xf_query)) {
tep_db_query("insert into " . TABLE_PRODUCTS_TO_PRODUCTS_EXTRA_FIELDS . " (products_id, products_extra_fields_id, products_extra_fields_value) values ('" . (int)$dup_products_id . "', '" . tep_db_input($xfield['products_extra_fields_id']) . "', '" . tep_db_input($xfield['products_extra_fields_value']) . "')");
}
// EOF: Extra fields to database on duplicate