Ciruit Board with traces on top and bottom?
For lower volume PC boards manufactured in the U.S. or Canada, it is usually much, much cheaper to just use a board house' cheapest and most standard setup: 2 layers, 0.0625" thick FR4 board, green soldermask, standard drill sizes, etc. This way, they don't have to change the work flow. If that is the case, then you get the bottom side for "free" so you might as well use it to decrease resistance and improve reliability.

Duplicating the top layer is also a no-brainer. Don't bother laying out the bottom layer. Just copy the top gerber file and rename it as the bottom gerber file. Any text will read backwards though :)

You can use a text merge program (used by SW dudes) to see if there is any difference between the top and bottom gerber files. They are simple text files. If exactly the same, this is probably what the previous engineer did.