My great-great grandfather came over in 1912. Am I am British citizen?
Sorry, British Common Law does not trump US law, specifically Title 8 which defines what a US Citizen is. He may have inherited dual citizenship due to this, but it doesn’t override US Citizenship if he meets the requirements under Title 8.
Nope, but if your great great grandfather hadn’t made himself an American Citizen, your great grandfather wouldn’t have been.