The standard feedback text is confusing. It says that the first 100% answering option is correct even if the second correct option is marked by a student.

Create a MC question
Set to single answer option
Create more than two answers
Set two answers to 100%
Save settings and start preview mode
Mark first correct answer
Mark second correct answer
Both answers are marked as correct. If second answer is marked the standard feedback says that the the first answer is correct.