If you sealed the stained door with anything, then you have to use a paint and varnish remover to remove it before restaining…
If you did not, then just a light sanding before restaining will do… but you must use a stain that is at least as dark as the original (can't lighten up the stain)…
If the original stain and finish is in good shape, no peeling or flaking, you can restain over it.
Prep is the key to getting the new stain to stick. The door must be very clean. This is accomplished by wiping it down with mild soap/water and a soft cloth, regular dish soap works well, and rinsing with fresh water to remove any soap residue. After the first cleaning wiping the door with Paso deglosser is a good way to make sure it is truly clean.
Generally no sanding is needed as this can destroy to doors embossed wood grain.
Dave is correct that you can only use the same or darker stain. If you want lighter then a complete strip of the current finish and stain is needed.
Check out <a href="http://www.house-painting-info.com/staining-a-fiberglass-door.html">Staining a Fiberglass Door</a> for more information.