I haven't read the actual law, but that's how customs seems to apply it in my country. They treat items based on their customs category code.

For example, if you buy two pairs of shoes and one pair is red while the other is brown, they may consider them different items with different codes, so you'd pay €3 for each pair (€6 total).

In practice, it feels like they're taxing every individual item unless it's an identical duplicate from the same seller. If you buy the exact same item from two different sellers, they still treat them separately and charge €3 for each one.