A 145

As a general rule, spells don't stack with themselves. Adding a layer of grease certainly won't render the floor any more slippery.

The specific rule covering this case can be found in the Combining Magical Effects section, probably this one:

Quote Originally Posted by SRD
In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the best one applies.