I had it happen once, a replacement came via UPS in a couple days. A week or so after that, USPS delivered the original.
Yep, some lazy postal worker apparently didn't want to do his deliveries and must have marked something as undeliverable, but then eventually got around to delivering anyway. That's the only thing that makes sense to me, anyway.
They were MTB pedals and the bike shop that was selling through Amazon told me to just keep them both for my inconvenience. Thanks for being incompetent, USPS!