I know the nurse
I, Willy De Bouvrie, say :
conozco a la enfermera.
But duolingo says : yo conozco al enfermero
Nurse = masculino y feminino !!
When Duo marks you wrong, and your answer is correct, you can use the "Report" button/link to report it. Course contributors don't see every forum post but they DO see the reports.
Yes, many occupations in Spanish have both masculine and feminine forms.