Citing a paper published multiple times
This may be a special feature of CS or not. We depend a lot on conferences for publishing work. The conferences usually publish proceedings. The first version is distributed at the conference to attendees and may have a limited run. It typically has the conference name on the cover. In the case cited, SIGPLAN, as is typical, will put a separate cover on the proceedings, turn it into a (regular) issue of the Notices and distribute that to all members whether they attend or not. It is really a service to the community, since not everyone can attend an important conference. It is possible that corrections are made between the two versions, but I don't think that is common. 

If they are identical, you can cite either one with confidence that readers needing to follow the citation will find the paper.