Hello Carlos,
Since yesterday the number of similar arcs warnings has increased very much, but when you look at the supposed wrong data, they are correct and there are no recent changes. May be recent commits have introduced some bug in this regard. You can have a look at this one for example: 2010/01/21 09:14:35 ADVERTENCIA (RouteNode): Similar arcs ((N-630 Carretera de Gijón a Sevilla, http://www.openstreetmap.org/browse/way/34954325) and (N-630 Carretera de Gijón a Sevilla, http://www.openstreetmap.org/browse/way/34954325)) from http://www.openstreetmap.org/?mlat=38.32804&mlon=-6.32094&zoom=17
That warning occurs when you have multiple arcs that have the same start and end points and the same length. It could occur when there are duplicate ways in the map data but also it could occur if the style rules you are using generate multiple routable lines from the same way. I notice that in the above message the OSM ids are the same. That implies that you are generating two roads from one way. It's either a bug in mkgmap or in your style rules. Mark