This is my approach of how to disable commandButton until first click is processed and re-enable after first click results are returned from controller to page.

vf.page

<apex:page>
    <script>
        var defaultEventListenerForSubmitBtn;
    </script>
    <apex:outputPanel id="op-panel">
    </apex:outputPanel>
    <apex:commandButton value="Submit" 
                        action="{!controllerActionMethod}" 
                        rerender="op-panel"  
                        onclick="this.value='Processing..';defaultEventListenerForSubmitBtn = this.onclick;this.onclick=function(){this.value='Processing..';return false;}" 
                        oncomplete="this.onclick = defaultEventListenerForSubmitBtn;this.value='Submit';"/>
</apex:page>