Found a figure duplication problem in an already published paper
You should definitively discuss this with your mentor, who presumably knows the conventions of your field. If conventions or journal policy are still unclear at that point you can always check with an editor of the journal what course of action they do prefer.
Take the following with a dose of salt, because my physics perspective probably isn't to be trusted in your case given that you linked to a medical journal. However, I'd think that if you can repeat the experiment (with proper approvals in place) to fix the mistake, and the mistake doesn't invalidate the main conclusion, then an erratum/corrigendum is probably the better outcome for all parties. The International Committee of Medical Journal Editors has some guidelines on how journals should handle corrections and retractions, which  seems to agree with my position:

Pervasive errors can result from a coding problem or a miscalculation and may result in extensive inaccuracies throughout an article. If such errors do not change the direction or significance of the results, interpretations, and conclusions of the article, a correction should be published that follows the minimum standards noted above.
Errors serious enough to invalidate a paper's results and conclusions may require retraction. However, retraction with republication (also referred to as “replacement”) can be considered in cases where honest error (e.g., a misclassification or miscalculation) leads to a major change in the direction or significance of the results, interpretations, and conclusions.

If even pervasive errors can be handled as an erratum, probably your case can too. Finally, here's an example of an erratum fixing a rather large image duplication problem by repeating the experiments. Also see this Retraction Watch post.