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Hi Mark,
Can you please check to see whether this location is situated very close to the corner of a tile? I am guessing that the coastline cuts the corner of a tile and so you end up with nodes on each of the tile's edges at the corner.
Like I wrote in the message you replied to, it is at the tile border, lat=62.226562. If you go to the browse URL, the marker is very slightly SSW of a node of http://www.openstreetmap.org/browse/way/23553585 (the coastline). I would guess that this is some rounding issue. Initially, mkgmap thinks that the pre-existing node is not at the tile border and adds a node there, and later it notices that the coordinates are equal after all.
Yeah, I know it's on the border but is it at a tile corner?
Sorry, no, it is not. The tile corners are more than 100 km to the west and east.
The patch I posted does (read should) fix a bug in the sea code because when it's making up the edges of the land mass that is clipped at the tile's edge it doesn't notice if the coastline reaches the tile edge exactly at a corner of the tile. In that (albeit unlikely) case, the existing code would generate a consecutive point and you would get a message like the above.
I did not look at the map output yet. So far, I haven't enabled generate-sea in my "release" maps, because last time I checked, the output was unusable due to flooding. I am planning to enable it near term. Marko