Should I refuse the invitation to review a paper which I recommended for rejection? What should I do if I suspected who wrote this paper? 1) I have been invited to review the revision of a paper that I recommended for rejection. I cannot see the updated paper before accepting or declining this second reviewing invitation. 
Would it be ethical to review it a second time if I advised it to be rejected and the editor overrode my recommendation?

2) Time has elapsed, and now I am almost sure of knowing at least one of the potential authors of this paper (Double-blinded review). It turns out that I might have a conflict of interest with this person. So my first ethical instinct is to decline the revision of this paper because of this alone.

Any comments much appreciated, thank you.
It's fine to review a paper that you have previously recommended for rejection (assuming you gave reasons rather than just dismissed it). The editor took your comments into consideration when deciding to seek a revision and I would expect the authors to respond to your concerns in their revision.

However, the second question is definitely grounds for refusing. The editor may wish to include you because you raised concerns and are in the best position to assess the extent to which those concerns have been addressed, and you may be incorrect in your authorship guess.

What you could do is an email to the editor along the lines of "Since providing the previous review, I have come to believe that PERSON X is one of the authors and I have a conflict of interest if that is true. Unfortunately, I must therefore decline to review unless you can assure me that PERSON X is not one of the authors."