One example I remember for a time was the 1530-ish 219 Midhurst Convent School to Aldershot (Alder Valley).
This was supposed to be a double-decker (you could tell that, as it was routed to avoid the low bridge at Shottermill), but for a time (summer/autumn 1984) was frequently operated by two single-deckers instead, the second I think being a duplicate.
It was always a bit puzzling, this service. The diagram number for the regular bus (not the duplicate) was H15 (Hindhead 15), which would indicate single-decker (as double-deckers were in the H101-110 range) yet it was routed to avoid Shottermill bridge. On one occasion it was in fact a double-decker. On another occasion it was one single-decker. And on other occasions it had a duplicate, as noted above.
From January 1985 it was always a double decker (diagram H104, which went empty to Midhurst all the way from Godalming, IIRC, having worked various Guildford-area services earlier in the day).