This seems to be different to my case, and to another case I found with the same error. In both those cases, there were duplicate entries found - the lookup service returned two entries with identical url/address values, and that was the problem. Returning multiple vcenter instances should not in itself be a problem.
Is it possible there is a version mismatch? If you were installing SRM 6.1, you would need vCenters to be at 6.0 Update 1.
