Hi,
Edit the file "step" via the menu Display>views and change the line
echo $this->cart->displayButton($checkout_next_button,'next',$this->params, hikashop_completeLink('checkout&task=step&step='.$this->step+1),'if(hikashopCheckChangeForm(\'order\',\'hikashop_checkout_form\')){ if(hikashopCheckMethods()){ document.getElementById(\'hikashop_validate\').value=1; document.forms[\'hikashop_checkout_form\'].submit();}} return false;','id="hikashop_checkout_next_button"');
to:
echo $this->cart->displayButton($checkout_next_button,'next',$this->params, hikashop_completeLink('checkout&task=step&step='.$this->step+1),'if(hikashopCheckChangeForm(\'order\',\'hikashop_checkout_form\')){ if(hikashopCheckMethods()){ document.getElementById(\'hikashop_validate\').value=1; this.disabled = true; document.forms[\'hikashop_checkout_form\'].submit();}} return false;','id="hikashop_checkout_next_button"');
and that will disable the button after the user clicks on it and make sure that he cannot click twice on it.