It is quite valid for one property to have multiple street addresses and therefore postcodes. It could genuinely have frontage in two streets, or it could be the result of joining two properties in the same street that originally had different postcodes – many long streets have multiple postcodes along their length¹.
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[1] For example even a not-very-long street I used to live on, Alma Terrace in York, has three postcodes: YO10 4DJ on one side of most of it, YO10 4DL on the other side of that, and YO10 4DQ for both sides of the part between that and Fulford Road. I suspect from the street layout that the third code is due to that part of the road being added later, or originally not having anything on it needing a postcode.
In London what ends up happening is that many buildings are so large (e.g. St. George's Wharf[0]) that the household count restriction ends up with multiple codes being allocated.
And it's also valid for UK postcodes to be reused, e.g. following demolition of the original buildings, e.g. tower blocks, the postcode may be deactivated for a period of time, then reactivated when new buildings elsewhere need a new postcode.
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[1] For example even a not-very-long street I used to live on, Alma Terrace in York, has three postcodes: YO10 4DJ on one side of most of it, YO10 4DL on the other side of that, and YO10 4DQ for both sides of the part between that and Fulford Road. I suspect from the street layout that the third code is due to that part of the road being added later, or originally not having anything on it needing a postcode.