What to do when an editor informed that a manuscript submitted to their journal has also been submitted to other journals at the same time. The COPE's flowchart offers editors a step by step process when handling the issue, including action to take and who to contact.

Editors may be prompted by another editor or reviewer, or by text matching software, of potential simultaneous submission of a manuscript. The review should be suspended while an investigation takes place. Manuscripts should be compared to identify how much content overlaps. Where there is evidence of systematic redundancy or overlap, indicating repeated dishonest or fraudulent practices, refer to COPE guidance on systematic manipulation of the publication process. A journal's instructions to authors should include a policy on concurrent submissions.

Source: https://publicationethics.org/resources/flowcharts/submitting-manuscript-multiple-journals