Advice on-line seems to be that it is not a huge problem if there was also diesel in the tank, as the petrol sits on top of the diesel.
If you don't have breakdown cover that covers this and can't find a local service to do it (I see your location is in Sweden) you'll need to syphon the fuel out of the tank using a flexible pipe, dispose of it in a safe and environmentally friendly way, and refill with diesel.
Maybe the petrol station can help you.
Often cars have anti-theft devices to stop you getting a pipe into the fuel tank, I don't know if the duster does.
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