Term for a person who disagrees but says the same thing

Solution 1:

A person who argues for the sake of arguing, who prefers controversy and dispute to discussion and agreement, may be called eristic. A more modern term which could be used in this context is denier. Of course, this type of person could also be called disagreeable.

Solution 2:

I have often heard people say "We're vehemently agreeing with each other." vehement: (1) zealous; ardent; impassioned: (2) characterized by rancor or anger; violent:

http://dictionary.reference.com/browse/vehemently

Another common phrase is "You're preaching to the choir" referring to a pastor directing his sermon to the people in the building who most agree with him.