Parents of Alfie and Harry McQuillan describe what happened when they discovered the overdose of morphine had been given.
One twin was given ten times the recommended dose while the other twin was given 17 times the normal dose.
Both died soon after the drug was given.
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Wahid Rob mounted the curb in November last year, injuring four people.
A spokesperson for West Midlands Ambulance Service has confirmed one person has died and two more are in hospital after the collision.
A man has been charged with the murder of his wife, who died after being stabbed at the family home.