I recently came to know that the expression just desserts ( a punishment or reward that a person deserves) should actually be written with a single s, as just deserts.
The noun desert in this sense means 'something which is deserved'. However, it is pronounced the same as dessert, i.e. with the stress on the second syllable and this is why the confusion arises. The noun desert in the commonly used sense of 'dry, barren region' is pronounced with the stress on the first syllable.
You can read Michael Quinion's explanation here.