How did you check for duplicates? There is a tool in the description of this group.
I don't see a duplicate but I like to find duplicates with formulas. Are there rows in the table below what you shared?
Is this a form that lots of people will open at the same time? If so, each person will see the options available when they open the file. That can allow more people to registrar for the same class. It is best to share a link to the form with the people completing the form at different times.
I see that the limit was reached for row 5 and the value replaced with a string. This string needs to be different for each row. If not, a duplicate will occur when two or more rows exceed capacity.
Is the sheet with the responses in the same spreadsheet with the table?
Try to open FormRanger and check the settings. Turn off the triggers and back on.