I have the following navigation where .topNav has position:relative and subnav has position:absolute. I cant get the sublist to appear over the main list due to z-index prob
Here's a very good article that explains the stacking issues that machineghost mentions.
http://css-discuss.incutio.com/wiki/Overlapping_And_ZIndex
What you might want to consider (depending on why you're wanting the positioning on multiple elements) is adding a hover selector to .base (use JavaScript for IE6) that adds the class to give it relativity.
.base:hover{position:relative;}
This then means that the second .base doesn't have position: relative.