Ok, not an O over sideways O, nor a G over sideways G, but I have just listed on eBay an 1874H farthing with C over C as per wybrit's post above, if anyone's interested, with this link...
https://www.ebay.co.uk/itm/284069502338
I have another one myself, so this one listed is a duplicate, but I don't know how many more have been noted? Not as spectacular as the sideways double entries, but it's there if anyone wants it as a variety!