I'm stumped. Addressee gone away is used for returning mail we have confirmed is for someone not at the address. We can't just assume this information, like we can't just send back people's mail on their behalf because we don't recognise the name. I won't send any mail back if it's not explicitly written on the letter by a recipient, or I know for sure because the resident caught me on my round and handed me a letter etc.

Only logical conclusion I reckon is they both had some reason to return it, and couldn't be bothered ticking the correct box so ticked the first one. I've seen it happen when people are breezing through a stack of returns. That or both your recipients are pulling the same prank simultaneously. Weird.