If the paint you want to paint over is a semi-gloss or gloss, you really should sand the surface to roughen it so that the next coat sticks better.
What's most important is that you scrape off any loose paint and sand the edges to eliminate any "ridges" that might otherwise be seen between the old paint and wood.
Test to find out what kind of paint you have. Take a white tissue and wet it with acetone or nail polish remover, and clean that paint in an inconspicuous area. If it's a latex, it will dissolve readily in acetone, and the tissue will quickly take on the colour of the porch. If it's an oil based paint, acetone will dissolve it much more slowly.