Should I accept to review a re-submission to a different conference

Yes, accept. You can start your review by stating:

I reviewed version A of this paper @ [previous conference]. The differences between versions A and B are quite minor. My review remains largely unchanged:

You can then provide a revision of your previous review.

Did you receive the full manuscript or just an abstract when you were invited to review? In some fields, potential reviewers are sent the abstract alone, and based on this they choose whether to review. Only if the reviewers who consent are sent the full manuscript.

If this matches your case, you should go ahead and review. If not, consider the following:

(1) Did most of your review comments in the first review deal with accuracy and use of good scientific methods?, or,

(2) Did most of your review comments in the first review deal with originality, scope and relevance?

If the answer to (1) is yes, then you should review, and mention your previous review as suggested by @user2768. The reason is simple- if there are factual mistakes, they ought to be pointed out, irrespective of history. In fact, it shows that the authors are trying to conceal mistakes/shortcomings and hoping to get lucky with a lenient review.

If the answer to (2) is yes, then maybe forget about the first review, and evaluate how different the two conferences are. It is possible that one may demand greater originality and the other may demand greater rigour, and so on. Similarly, scope and relevance expectations could be quite different. If you find yourself unable to evaluate this, possibly you are being biased by your previous review, and you should consider declining the review. Otherwise, by all means, accept the assignment.