How Alike Are Two Tag Lists?

A photo site suggests similar photos by comparing their tags. It scores two tag lists like this:

score = (number of different tags they share) ÷ (number of different tags used by either)

So 1.0 means identical tags, and 0.0 means nothing in common.

Task: write similarity(tags_a, tags_b). Return the score rounded to 2 decimal places.

• Repeats inside one list don't count twice.
• If both lists are empty, return 0.0.