i think “一般般”and“一般'" the two words have no difference when they are used to express something is very common. the diffence is that different people inclines to say diffent expression. the second difference is: "一般般“is only used individually,for example, when you say:the weather in beijing is generally good.(北京的天气一般很好），but you can not say:（北京的天气一般般很好）。