An easy way to do this is to add validation. When editing the question, you can select the Validation Type "Must Rank All" or "Must Rank Between." Both of those options will automatically make sure that rankings don't have a tie. Naturally, the "Must Rank All" option will require that respondents rank ALL of the items, as well.
If you'd rather not force your respondents to rank every single item, then use "Must Rank Between." You can set the boundaries to at least 0 and no more than however many items you have in your question. Here's an example in the screenshot below. I only had 5 items in my question, so I specified that respondents should not rank more than 5 items. This validation type is also useful if you only want respondents to rank their top 3 out of a longer list, and so on.