There are a few like that in this area. In each case, a block was divided into building lots for sale with the lots numbered. Much later, an owner subdivided his lot and built a second house, so there was no other option than to number that lot and house by adding 1/2 to the original lot number. This would all have occurred prior to the municipalities adopting zoning codes.
Yes, there are quite a few in these parts. Its usually when a house is turned into flats or if a garage is converted into a house. Near my granddaughters school there are large houses with a large piece of land in between the house next door and couple of them have new houses built in between so they're numbered 12. 12 1/2 then 13 and 13 1/2. The postmen get confused a lot.
Sometimes around here, when they divide up a house into flats, they add letters after the address.
Example, 128-B Monroe ave.128-C, etc.
I haven't ever seen 1/2 on anything.