The issue doesn't arise when the room only has a capacity of 1, as it's not possible to book such a room for 2 guests.
The issue is that for a room which has a capacity of 2, many Hiltons charge slightly higher rates when you put 2 guests in the booking versus 1.
This is more common in mainland Europe than the UK but in the UK I note that the Hampton Corby does it, because they consider the breakfast "included" rather than "free" and therefore it has a notional charge of £8pp.
If two guests turn up having booked a room with a capacity of one, they (well one person) will be refused check in, regardless of 2nd guest stays free - they will be forced to upgrade to a room that sleeps 2 and probably the check in staff may not know to quote the single occupancy rate for the double room.
I'm guessing there aren't different rates for redemption stays based on single occupancy though!!
Although if diamond they may have upgraded you for free anyway, which is what happened to me when I booked a prepaid single room. My wife later decided to join and we decided to chance it, I got upgraded to a large room so there was no problem in the end and I didn't have to pay extra (except the city per person tax).
Thank you very much for posting this. I very often stay on my own and have never thought to change the number of guests on the app before booking, so I will do that from now on. I've got a few days at various properties in Munich coming up for instance. I can also see the point about the included breakfast at the Hampton being considered differently than at (say) a HGI because it is not an elite benefit.
Thank you!!
EDITED to add that I checked my settings and it seems I have been searching for single occupancy all along!!! (1 room, 1 guest).