Q: Is conducting a similar research with additional data considered a duplicate publication?

As you have mentioned, the research that you are planning to conduct will contain only additional number of cases and would not involve research on any new aspect of your topic. Publishing such a study would compromise the novelty aspect required by journals and your research might be considered redundant if not duplicate, and it will be difficult to convince a journal editor to publish it.

Simply adding a number of samples or cases to your study is not sufficient. You also need to have a different research question in the second paper you are planning to write. If you choose to go ahead with this idea, you will have to cite the original paper as well as inform the editor of the journal of the previously published paper.

However, if you do believe that adding a few more cases is important and will make significant contribution to your field, it would be a good idea to write to the journal in which you published the original paper and ask them what would be the best format to publish your latest findings.  Based on what the journal says, you could publish your latest research as a follow-up study in the same journal.

Related reading:

• Is adding literature review of a published paper in another manuscript considered duplicate publication?
• Can two papers with same data but different analysis be considered duplicate publication?
• Duplicate publications and simultaneous submissions