There are a couple ways you can go about repainting your doors. I do not recommend painting latex over old latex. The first way you can do this would be to sand it down to rough up the paint to ensure a good bond between the two paints.

The second way would be to prime the door, Which is prob the way I would go, with some light sanding to remove and runs or drips from the previous Paint project. This is the greatest percentage of success and worry free method.