I could be wrong, but I always thought a dg specialist was an entirely separate position (pay grade and all), and unless its specified in your contract that you will be responsible for those extra duties, you can refuse to do it. The whole purpose of the contract stating it is because if you accept the position you are basically forewarned and willing to take on those extra duties at the specified pay...otherwise you are performing extra work without being compensated fairly for it. Am I wrong?
On a side note...if its not specified as a requirement of your position on the contract you signed, and you really don't want to do it you could always just fail out on purpose...problem solved.