How to write a statement of research interest for postdoc positions that already have a defineed research direction I'm currently applying for a postdoc position in a particular lab. The lab has several postdoc positions available at this time, those postdocs will work on different aspects of a larger research project.The application need to include statement of research interest, CV, references and recommendation letters in one document, and also clearly state which position (which aspect of the research) I apply for.

I feel I'm a good fit for two of the positions. However, I don't know how to write a statement of research interest that is suitable for both positions, although those two aspects of the research are connected. From all the examples I saw for research statement, future research is a big part of it. With the research direction and aspects are clearly defined for the positions, what should I write in the section of future research?
One way is to write an SOP that is a bit more general then you would do otherwise, but not so general as to seem unserious. If you do it carefully, it can be an advantage. I think that an SOP too tailored to a particular position might not be taken seriously -- too good to be true. But don't go to the other extreme of suggesting that your interests are so broad that you don't/can't specialize as needed. 

I wouldn't recommend this for positions that were widely different, but it might be good here, as you say they are related. 

But, as you say, the direction of the lab is set and a post-doc isn't likely to change that. If your research interests are aligned, your "future research" ideas will probably fit as long as you don't overemphasize the specifics more than needed. 

This is a Three Bears sort of problem. Too general. Too specific. No, just right.