How should I decline a referee request for the paper which I refereed before? I have received a referee request for a paper that I refereed and rejected a few months ago. I would like to decline the request because although the recent journal's rank is lower than the previous one, I will reject the paper again for the same reasons I gave last time. Hence it would be better to decline the request to referee. What should I write to the editor in this situation? 

A friend suggested I write to the editor.
I recommend against declining. You've already refereed the paper once, which means you can referee the paper again much quicker than a fresh set of eyes can.

Instead, send your original comments back. If the authors have updated the paper, send an explanation of why it's still not publishable as well. Then write in the "confidential comments to editor" box that you've reviewed the paper before, you still recommend rejection, and your original comments are attached. The editors will know what to do.