Old listeners = {'BOB', 'RYAN', 'CARL', 'FURGUS', 'CALEB', 'AMANDA', 'KATE', 'LUKE', 'FRED', 'JOHN'}
"New" listeners = ['LUKE', 'RYAN', 'ERIC', 'CALEB', 'MELISA']
My answer = {'BOB', 'RYAN', 'CARL', 'MELISA', 'FURGUS', 'CALEB', 'AMANDA', 'ERIC', 'KATE', 'LUKE', 'FRED', 'JOHN'}

The correct answer is 5, but the new listeners are only ERIC and MELISA (they didn't listen the song), and LUKE, RYAN and CALEB have already listen the song, so I return as answer 2 but is wrong.

I don't understand, can you give an example.

Some randome tests in python have issues, they have already the name and they except it as a new listener.