Ha! Good eye. I didn't even notice the doubled-up 922526. It's not a FP bug. Rather, it's a corner case thanks to how I combined the /2 optimization + memoization; certain even numbers get added to the p.queue twice. It can be fixed trivially by either removing the /2 optimization (simpler, ~5% penalty), or skipping seen numbers in the second call to top() (no measurable penalty, adds a variable).

As to your interpretation of the "top 20 arrangement," I like your discussion! We try to keep the task descriptions only quasi-formal, to keep the challenge accessible to beginners, which is why you don't usually see these sorts of details specified like a requirements document. Meaning, many "minor" details are left to the discretion of the participants. The upshot of that is, if you submit a really weird interpretation, you'd probably net yourself a mildly amusing comment in my next review, at least. :-)