Double publication is considered to be a major ethical misconduct (as it is mentioned in nearly all journal licenses during manuscript submission). You should do the rightful regardless of the knowledge of the authors concerning your identity and notify this to the editor as suggested by @Wrzlprmft.
But, before any hasty action, ensure whether the manuscript is an exact replica of the authors' previous publication. It is customary for authors to improve their methods over their past publications and publish them in another journal for better visibility. Such an act is not to be confused with "double submission". Nevertheless, the reference of the article suspected of double submission ought to be reported to the editor too.
Most reputed journal publishers use proprietary plagiarism tools that could also detect double publication. TBH, I guess the journal editor might have access to such tools (such a situation would not have risen otherwise). Which does give rise to another question of reliability of the journal itself. SpringerPlus is part of the SpringerOpen series, it is surprising that an editor from such a reputed publisher would provide a manuscript possible of double publication to an unsuspecting reviewer. It could be a security threat - a phishing attack to be precise. Does the mail provide any link other than the official site to submit the review result or for any other purpose? If so then it would be best to not to even reply to the sender. The rightful thing to do would be to contact the board of SpringerPlus mentioned in the site and report the incident to ascertain the reliability of your mail.