You can add some validation to your question. Click on your ranking question to edit it, and select the validation type (screenshot below).
It doesn't need to be custom validation. If you pick either "Must rank all" or "Must rank between" then respondents will not be able to proceed with duplicated rankings. If you pick "Must rank between," then you can set the minimum as "1" (respondents must rank at least one of the options) and set the maximum at whatever you like - or even leave it blank.

Thanks, @lizb311! That worked!

algroom_heart Awesome! Glad it worked for you