Layouts, styles etc will not really hurt you...what will is the duplicate content if the majority of it is identical to another. What happens is that the bots will give credit to the first one that had it and you will lose any efforts on the ones that do have duplicates with. It's also good practice to make sure your sites code is valid (among other things -- a list that would be big if I typed it all here).
The bots will find you no matter where you hide duplicates...lol
