Resusing my own poster abstract for another conference
Assuming you're talking about a one or two paragraph abstract, the question here is not really about the abstract per se but the contents of the poster.  In my opinion, you don't need to worry about self-plagiarism per se from a brief poster abstract because it's not a real publication in the first place (if it were a 1+ page extended abstract, that would be a different matter).  

Instead, the important question is whether your research community will find it acceptable to have you presenting essentially the same material in two different conferences.  In many communities, this is OK, taking the view that it is essentially raising visibility of a result with different groups of people; in others communities, however, it may not be viewed as acceptable.  Consult with your supervisor to see what the standard is for your community.