COPE's guidance, as a flowchart, on what to do if you suspect redundant (duplicate) publication in a published manuscript.
The instructions to authors should state the journal’s policy on redundant publication.
Asking authors to sign a statement or tick a box may be helpful in subsequent investigations.
ICMJE advises that translations are acceptable but MUST reference the original. Editors may consider publishing a correction (i.e. the link to the original article) rather than a retraction/notice of duplicate publication in such cases.