It's very common for any boat fill (gas or diesel) to quickly overfill as they don't (newer ones usually do, though) automatically shutoff like a car. Add to the mix that diesel pumps flow faster than gas and filling up a small tank happens REALLY quickly. Slow down and listen for the "gurgle" noise from the vent - that's when the fuel is near the top. Alternatively, you could install a whistler into the vent to make it easier to hear when full.