Powder coating is tough and lasts a long time when not exposed to strong forces that could chip it. Unfortunately, most powder-coated items are exposed to said forces at least once, if not more often. You've likely seen the effects of these forces; think of the powder-coated stairway banisters outside a building on a college campus with all their chips and divots from the occasional skateboard trick. There are ways to repair damaged powder coating, and which one you choose depends on the severity of the damage.
Patching for Small Chips and Scratches
Very small pits and scratches can be covered rather easily. The finished surface might not look as smooth as it did when it was freshly powder-coated, but the item will be fully coated with no spots where rust or other corrosion can take hold. Paint is often used, or you could find a sealant to protect the exposed material.
You can do the same for larger, isolated areas where the powder coating has been chipped away. You may be able to get the area sprayed with another layer of powder coating if it is big enough.
Coating Over the Existing Coating
If the item is damaged in several spots, you'll need to recover the entire item, and you can do this in two ways. One is to simply powder-coat again over everything. Some companies are not willing to do this, but others are. You won't see a smooth surface, but if all you're concerned about is covering a material like metal to prevent it from rusting or to otherwise insulate it, a quick re-coating could be an option.
Stripping Away Coating and Re-coating
The other option, and the one that most powder-coating companies will prefer, is to have the existing coat stripped away, such as by sandblasting, and then give the item a fresh powder-coat. This is absolutely your best option if looks are a concern. For example, if you need to powder-coat something that will be on display where many people can see it, sandblast the item to remove the current coating and give it a fresh coat.
It can be difficult to protect powder-coated items from everything, particularly if they are in public areas — skateboards, people absentmindedly picking at an already exposed edge of coating, and so on — so start asking powder-coating companies what they can do should you need to fix a flaw or some damage. That way, you'll already know what you need to do should you spot damage.