Form is only submitted once

Hi,

I have a problem with creating forms. My code (just relevant parts, as I think):

function user_gallery_menu($may_cache) {
	$items[] = array(
			'path' 		=> 	'usergallery/admin',
			'title'		=>	t('adminarea images'),
			'callback' 	=> 	'_user_gallery_admin',
			'access' 	=> 	true,
			'type' 		=> 	MENU_NORMAL_ITEM
	);	
	return $items;
}

function _user_gallery_admin() {
        $themeingstring=theme(...);
	return drupal_get_form('user_gallery_frm').$themingstring;
}

function user_gallery_frm() {
	$form=array();
	$form['#attributes'] = array("enctype" => "multipart/form-data");
	$form['image'] = array('#type' => 'file', '#title' => t('Image'), '#weight' => -3);
	$form['submit']=array('#type'=> 'submit', '#value'=>'Hochladen');
	return $form;
}

function user_gallery_frm_validate($form_id, $form_values) {
//validation
}

function user_gallery_frm_submit($form_id, $form_values) {
    drupal_set_message('submitted');
   cache_clear_all();
}

After submitting the form the first time, the message "submitted" appears as I wanted. After submitting the same form again (where I have been redirected to), no message appears. And it seems, neither the validation nor the submit-function are called. So I entertained suspicion, that the page was cached. But when I turned off caching, it is still not submiting.

When I rename the complete formname and its hooks(for example "user_gallery_frm1"), the same is happening. It submits the form once and the next time it wont submit.

Can anybody help me, what I did wrong?

Thanks in advance.

Markus