Safe laboratory practice requires accurate patient identification. Adverse events may occur when a patient has identifiers similar or identical to those of another patient (a 'doppelgänger'), is doubly registered (a 'duplicate registration'), or when registration details are derived from two or more separate sources (a 'hybrid registration'). Distinguishing doppelgängers from duplicate registrations is not always easy. A search of the Harefield Hospital Patient Administration System (PAS) database revealed 39 registrations that shared a forename, surname and date of birth with at least one other registration. Thirty-seven of these cases involved a duplicate registration, one involved a hybrid registration, and one involved a doppelgänger. The National Strategic Tracing Service can help with resolution of difficult cases. Many serious patient identification errors involve what the Serious Hazards of Transfusion (SHOT) Report refers to as 'extraordinary' coincidences of patients with similar names. Such coincidences are, in fact, not extraordinary, but ordinary. A major challenge will be to establish how adverse events involving coincidence can be described in a way that does not create the impression of extraordinary bad luck.