Are reviewers required to check for originality? Suppose a person writes a research paper without doing a complete literature survey, and sends it to a peer-reviewed journal. Assume that the results in question have been obtained before by someone else.  Is it mandatory for journal editors to check the novelty? If not and if the editors also don't know about the pre-existence of this research, then who is liable for the aftermath?
As I see it, the reviewers and editors don't have an obligation to redo the literature search.  However, the reviewer should be someone who is sufficiently familiar with the topic that they would be reasonably likely to know about relevant prior work, and should be able to identify serious omissions in the literature search. A reviewer who doesn't have that level of expertise should not be assigned, and if assigned should probably decline to review the paper.

Of course, if the reviewer does know about prior work that duplicates the paper under review, then they should require the authors to cite it, and evaluate the paper based on its novel contributions only, if any (or as an independent simultaneous discovery, if the timing makes that a possibility).

Final responsibility for the content of the paper always rests with the authors alone.  However, if the paper is accepted and later found to be a rediscovery, a common course of action is to print an "acknowledgement of priority", a short note citing the previous paper and acknowledging that it was first.  Typically it wouldn't be retracted.  (Of course this only applies if the editors believe the paper is really an honest independent rediscovery; if it is plagiarism then it should be retracted.)