SpringerPlus asked me to review a paper already published I have gathered online that SpringerPlus is legit. thing and not some scam. But they emailed me (a  PhD student) and called me a Professor and asked me to review an article which was accepted in some other journal. 

I feel like i should do the review because it's good for one's career, but this seems very strange. What to do?
You should immediately inform the editor and let them decide how to proceed further.

If this is indeed a double publication, reviewing it is a waste of time for you, other reviewers and the editor. In the unlikely event that there is some good reason for this (e.g., the other journal is a scam), the editor should be able to find out quickly or already know about it and tell you.

As for your reputation as a reviewer: If you see something very suspicious, reporting it is the proper action and is a job correctly done.

You should also ensure the following:


The other journal is really a journal and not, e.g., a preprint server like the ArXiv.
The mail asking you for a review did not mention this fact.
The mail asking you for a review really came from a reputable publisher (and did not only claim this).