Difference?? : 聊，聊天，聊聊，聊聊天，聊一聊，闲聊 What's the difference between 聊，聊天，聊聊，聊聊天，聊一聊，闲聊？
All of the words you have chosed have similar meaning to each other and in most cases can be used interchangeably. But I guess you knew that, because that's why you're asking.

All of the differences are very subtle, and the most notable ones are the differences in formality. There are also slight differences in the linguistics of usage, but I'm not familiar with much of that. Also, note that on the face of it, 聊 itself and its extensions are generally more informal compared to words like 谈 or 讨论. Its direct English translation is "to chat", and I think that that definition carries the notion of informality over quite nicely. Below, I'll discuss formality in levels, from 1 (most formal) to 3 (least formal).

Formality Level 1: 

聊 is the most general form; when you wish to chat with someone on no particular topic (or one you don't wish to disclose) you can say something like "我现在跟你去聊。" Compared to the other versions and extensions, this is generally more terse and laconic than the others, and thus more formal. It shares the same formality as 闲聊, although this one is used more when one wants to emphasize that it's a friendly and informal discussion with the addition of the 闲 in the beginning.

Formality Level 2:

聊天 takes the spot for an intermediate formality. It is slightly more informal than 聊 or 闲聊, but also more formal that the remaining few. This phrase, like 闲聊, emphasizes the "chatting" quality of your discussion. With its addition of 天, it references the weather, and links it to the fact that most people talk about the weather in a situation where you want to "catch-up" with someone. However, you don't have to talk of the weather when you use this phrase. Most of the time, both 聊 and 闲聊 can be interchangeable with it.

Formality Level 3

The remaining 聊聊, 聊聊天, and 聊一聊 are the least formal, and are common colloquialisms. We see it a lot in the Chinese language, but typically when one repeats a single-character verb like 聊 to 聊聊 or 说 to 说说 it shows informality, almost like talking to children (one very good example of this can be seen in the names of Chinese children; a lot of the time, children have nicknames that repeat a single-character, such as 茜茜, which is associated with a juvenile tone). 聊一聊 is also regarded in the same manner, used in sentences such as "我现在跟你去聊一聊！". However, out of all of the words, 聊聊天 is probably used the least, because the same informal effect can be accomplished with 聊聊 or 聊一聊, and there is already a 聊天 that is already popular in the mainstream.

Keep in mind that these are very subtle differences. My knowledge on this topic is merely from my own experience speaking Chinese in mainland China and Taiwan. If you are interested in even more linguistic subtleties or social conventions, consider talking to a Chinese language expert at your local university, or even looking into it yourself. That stuff is out of my scope.