Custom Validation for Rank Order Question

I would like to create custom validation for a rank order question. The rank order question presents a list of nine items. Respondents must give a ranking to all nine items on the list. One and only one of the items must be ranked number 1. However, ties (i.e., indifferences) are allowed below that. For example, after identifying their first choice, a respondent can rank two items as second choice, two items as third choice, three items as fourth choice, etc.

I do not know how to create custom validation to allow for this type of ranking scenario. It would be so much easier to just create a strict ranking, but I've been instructed to allow for indifferences after the first choice.

I was thinking that perhaps a pick/group/rank question might allow for this, but a simple rank order question with the custom validation doing the tough work of enforcing the constraints would be so much cleaner for the respondents.

I would appreciate any ideas you might have or guidance you can provide about how to create such a custom validation.

Thank you!