How did this happen, was it Jet-A1 from a pump? If hand-filled the fuel is so different in smell and viscosity it's hard to see how you would keep on filling after the first few seconds.
If it was Jet from a pump, wouldn't the the OP have called it Jet rather than diesel? But as you say the fuels are so different to handle. Very strange.
Not wishing to add to the chorus because we all make mistakes, but if such a basic error was made, how much else of the pre-flight wasn't done properly?