If one cell is enough, than one atom is enough and so on. Then, nothing's really the same. But we use this notion because it's convenient, so we're back at the beginning.
"Is a single grain a heap" doesn't look like a paradox to me. A "heap" is just a word with an ambiguous definition. Once you define it, you'll have your answer :)
>Then, nothing's really the same
If that was the case, intelligence would've been impossible. We need similarities as well as the differences to be able to compare things (including abstract concepts)
If one cell is enough, than one atom is enough and so on. Then, nothing's really the same. But we use this notion because it's convenient, so we're back at the beginning.