I am getting similar errors. Newly converted project to Unity 5. Downloaded 2.5 from the releases thread and updated...
Assets/TK2DROOT/tk2dTileMap/Code/tk2dTileMapBuilderUtil.cs(8,29): error CS0101: The namespace `tk2dRuntime.TileMap' already contains a definition for `BuilderUtil'
Assets/TK2DROOT/tk2dTileMap/Code/tk2dTileMapChunks.cs(8,22): error CS0101: The namespace `tk2dRuntime.TileMap' already contains a definition for `LayerSprites'
Assets/TK2DROOT/tk2dTileMap/Code/tk2dTileMapChunks.cs(18,22): error CS0101: The namespace `tk2dRuntime.TileMap' already contains a definition for `SpriteChunk'
Assets/TK2DROOT/tk2dTileMap/Code/tk2dTileMapChunks.cs(83,22): error CS0101: The namespace `tk2dRuntime.TileMap' already contains a definition for `SpriteChannel'
...
Actually, these errors seem to be present even when using my earlier version of 2DTK, when updated to Unity 5.
Any idea as to what may be causing these?