How to avoid Ext.Viewport duplicate instance.

I have main menu.. from there I call respective view buy uisng

Ext.Viewport.setActiveItem({xtype: 'viewalias'});

This works fine. But in my veiw I have come ComponentQuery for identifying some panels and buttons.
This also works fine for first time.. if you go back to main menu and come back agian.. then this ComponentQuery is not fetching / not working.

Suspecting the Ext.Viewport is creating duplicate instances.. and hence there is conflict.

Let me know is there alternative for Ext.Viewport.setActiveItem({xtype:.....}) or best way to use it.

Thanks
-Ram