The set of choices must be unique error appears on single choice, multiple choice, and dropdown fields. The actual text of the error is pretty self-explanatory. It appears when choices have the same structure.
Each choice is related to a column in the Submissions table based on an identification number. If choices are identical so is the identification number.
Therefore, our system is unable to separate them and returns the warning message.
The good news is that you can easily solve this error.
To fix it, go to your fields and ensure that you have different text choices.