Caryl Churchill's 2002 near-future play, sparked by the cloning of Dolly the Sheep, sets up confrontations between a father and three sons.
But in a dense 60 minutes, Lyndsey Turner's taut revival raises fewer questions about dystopian science, and more about individuality, nature versus nurture, and parental damage.
Bernard 1 (Paapa Essiedu) is upset to learn he's one of 'a number'. Father, Salter (Lennie James) placates him that he's the original, and much loved. Perhaps they should sue? Then Bernard 2 arrives; angry and resentful at being abandoned years earlier. In a revelation that has violent consequences, it emerges Salter put the neglected, damaged boy into care, then cloned him to "get it right" second time.
Possibly unauthorised, the scientist behind the cloning has copied 20 more, including a shallow, contented American who Salter fails to connect with in an excruciatingly awkward art gallery meeting.
Es Devlin's rectangular box set is a bland modern flat washed in angry red-orange, and Tim Lutkin's white-out bursts of light reveal unnerving quick-changes between the Bernards.
Essiedu and James masterfully deliver Churchill's densely-packed, elliptical exchanges with emotional realism and even humour, throwing up ideas about individuality and uniqueness.
But this also plays as a family tragedy of filial jealousy, revenge and atonement as Finsbury Park actor Essiedu brilliantly switches between vocal and physical registers, from the confused middle-class Bernard, who no longer knows who he is, to the edgy, disturbed original who at one point offers a traumatised, mute response to his father's apologies.
We know from Line of Duty that James can do slippery, but as he shifts between, defensive, evasive and pleading he also manages to retain a queasy empathy for Salter's weak desire to wipe the slate clean rather than fix his mistakes.
But while the pair exude unmistakable chemistry, James in his first stage part for years, doesn't always manage to project his understated performance to the back of the room.
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