Hi!
Just to make sure I understand correctly:
You ask: Please list the initials of the members in your household
If they say: AB,CB,DB
You say: It's all good!
If they say: AB,AB,DB
You say: Please enter unique values
If that is correct...
You will need to set up custom validation in your form question. Depending on how many answers they give (family members), you will need to create the appropriate number of conditionals. In this case:
Mom cannot match Dad | Dad cannot match Sister | Mom cannot match Sister
Note: You will need to get the piped text string in a separate question and then paste it into the custom validation.
Please let me know if this makes sense, or if you have additional questions!

Thanks, David!
This was very helpful!

Happy to help!