Putting Duplo lego sets or toys in the dishwasher is not recommended because the heat from the dishwasher heating element could melt them.
However I have washed Duplo and other lego sets in my dishwasher as well as some other plastic toys but I put the Duplo and plastic toys on the top rack of the dishwasher away from the heating element.
I've never had an issue with the Duplo , Lego sets or other plastic toys melting.
Just like I've washed plastic sippy cups, plastic baby bottles etc I've been able to safely wash the Duplo and other plastic toys in the dishwasher and its never been an issue.
Just don't put the Duplo on the bottom rack and they should be fine.
You can also just soak the Duplo in hot water in your kitchen sink or bathtub and wipe them down with a washcloth and rinse them off.