SpringerPlus asked me to review a paper already published
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).