Writing "thank you" note to professor who will write more LOR's?
If you are worried that sending such a letter would be something of a bribe for the future letters, you can rest easy. No one is likely to think that. But there may be a lot of reasons to thank professors, including what they contribute to your education. 

But in the case that someone writes you a letter and you get an award or offer partly as a result, then you should definitely thank them.