The "duck" is being compared to absolute nothingness. In mathematical terms, "duck" minus zero equals "duck". Whether or not nothing can realistically be compared with something is beyond the point. All one has to do is interpret the question within reason and conclude that you have a "duck" and you end with that one whole "duck" after comparing the differences.
This question also makes a grammatical error, which is obvious. People who like creating deepities sometimes pose a question that makes a mistake in grammar. I used quotation marks around the word "duck" because I was not talking about an actual duck but the concept.