Answer
The layer names repeat when one or more layers in the map are symbolized with transparency or BMP marker symbology.
ArcMap uses the Microsoft standard display and output pipeline, known as the Graphics Device Interface or GDI. The GDI does not support transparency or non-rectangular raster information; therefore, ArcMap must process maps that contain transparency and/or BMP marker symbology as raster.
Raster maps are processed in bands, so each time the layer name repeats, ArcMap is processing the data for another band of the output.