The point is, you are redeclaring a function, or PHP wouldn't say so.
Do you have any "include()" statements?
And, I know this sounds silly, but. Are you looking at the right source?
If you use shopping_cart.php as an Include() you will get the error at the point where you declare the function for the second time (in the shoppingcart.php)
But, the first time can be in a completely different source.