John, thank you for the reply. Currently, 9509 and 9509A belongs to two different households: Christine Pigge, and Nan Zhuo. They sometimes received each other's mail if someone forgot to put A before 9509 for Nan's house. It's a shame that OSM confuses them as well.

Umd.me.uk is registered under the 9509A address so has the right to include it in the house tags as well.

In fact, many houses have a website as well. For example, the following is a Tous Les Jours website for part of a building:

https://1.umd.me.uk/nominatim/details.php?osmtype=N&osmid=13861413701&class=shop