“Dead Ringers” (1988), by David Cronenberg, about Elliot (Elly) and Beverly (Bev) Mantle.
«- Something radical is definitely required. (…)
– Yes, Doctor!
– Can I trust you to do that, or have I got to sit here and watch you?
– I don’t know. Can you trust me?
– Oh, God! Don’t do this to me, Bev.
– But I’m only doing it to me, Elly. Don’t you have a will of your own? Why don’t you just go on with your very own life?
– Do you remember the original Siamese twins?
– Chang and Eng. They were joined at the chest.
– Remember how they died?
– Mmm. Chang… died of a stroke… in the middle of the night. He was always the sickly one. He was always the one who drank too much. When Eng woke up beside him… and found that his brother was dead… he died of fright… right there in the bed.
– Does that answer your question?
– Poor Elly.
– Poor Bev. (…)
– Look, don’t you get it yet? Whatever is in his bloodstream goes directly into mine. (…) Beverly and I just have to get synchronised. Once we’re synchronised, it’ll be easy.»
As a verb, ‘ring’ has long been used to mean ‘exchange/substitute’ in a variety of situations: for instance, ‘car ringing’, which is the replacing of the identification numbers on a stolen car with those from a genuine (usually scrapped) vehicle.
‘Dead’, in the sense of ‘exact’ or ‘precise’, which is demonstrated in many phrases: ‘dead shot’, ‘dead centre’, ‘dead heat’, etc.
So, ‘dead ringers’ are literally the same as ‘exact duplicates’.