Names of residential, unclassified ways are repeated too often at z17+.
As an example: http://imgur.com/mAOcMXx (This same area in OSM)
While trying to fix this, I found that in the OSM mapnik stylesheet, (Line 3226), residential and unclassified ways at z17+ have a text-spacing of 400px.
Since this is missing in the carto stylesheet other.mss at the moment , I added - text-spacing: 400; at line 2875 to other.mss in my local (and updated) repo but it didn't fix this.
I wondered if the catch-all (noted with comment as 'other roads') (written right below the rule) was the culprit, but I also added the text-spacing:400 there and the names were still close together.
I also made sure, each of those ways in the examples are each one singular way, not multiple ways connected together (that have the same tags).
Good catch. This is actually a line-clipping problem where mapnik now automatically clips the lines that it's using for text placement to the size of the metatile. Because the metatiles in TileMill are much smaller (2x2 instead of 8x8) it's noticeable in TileMill an much less so on mod_tile based servers.
Adding text-clip: false to the rules means that the text-spacing is better respected. We'll need to do the same for many other rules. Additionally you're right about residential z17+ being 400 instead of 300.
See also mapnik/mapnik#1608 and mapnik/mapnik#1569 (comment)
Fixed in 5a48086 (not sure where we added text-clip: false, but it's there too)