Yes, you're right. You used to be able to actually build the modules even if the resource was already being mined. Of course you couldn't get the resource while doing so until their starbase was destroyed. You can still get ready to take over that resource by doing just what you always did before, without the actual build part. Just click on the pass button and then after destroying their base, then click on the starbase again to build the appropriate module. I'm sure you already know this but I'm just outlining it for others that don't.
Yes, I used that little exploit all the time myself. 
