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Hello, I hope the following is of assistance.
The best thing to do if you haven't driven the car is to have it drained of fuel then refill it with the correct fuel. If you've driven it then you need to drain the fuel from the tank, clean the fuel lines and replace the fuel filter.
Petrol is a solvent that washes the diesel oil out of the fuel system, the diesel being an oil lubricates the fuel pump and injectors so leaving petrol in the system will cause damage to the fuel system. If too much petrol gets into the fuel system the engine will stop anyway, usually before this happens you'll get a bad diesel knock from the engine caused by the petrol detonating.
If you drain the system and refill with diesel then this should prevent any long term damage but if you run the car as it is you risk damage that will be expensive to repair and certainly more than the cost of draining the system and cleaning it.
I hope this helps. let me know if I can assist further and I'll reply as soon as I'm able to.