Dear Iuri,
in the corrent versions of OpenACS the order does not matter, as the following example shows:
ad_form -name foo ad_form -extend -name foo ad_form -name foo -extend ns_return 200 text/plain "fine"
This was fixed a half year ago or so in CVS and will be part of the next OpenACS release.
As always, please include version numbers in bug reports and report these on the bug tracker.