Unique fails on multiple values

I'm experiencing an unexpected behaviour (bug?) using Unique Field. I have a content type where I've set it to be Unique. The module works as expected as long as you only use a single value, but the field for the content type can have multiple values ("Add another item"). As soon as I add another item, Unique Field stops checking that field and let's it through, even though it's not unique.

Step 1: Create new content with some text in the field that's supposed to be unique. Save.

Step 2: Create another node and use the same text for the field. Unique Field flags it as not unique.

Step 3: Just press "Add another item", type something unique in the second field. Unique Field now doesn't fire any warning and non-unique value is saved to database.

Am I doing something wrong or missing something?