Created attachment 39655 [details] Reproducer For a conditional formatting rule, it is possible to define multiple areas while looking for duplications. In my reproducer, the area to check duplication is (A0;A1:H1). In Excel, if A0 and H1 have the same value, the rule triggers the conditional formatting. With Apache POI, areas A0 and A1:H1 are checked one after the other. The rule is then not triggered.
The POI community is pretty much moribund. Unfortunately, if you want a fix, you basically need to do it yourself. If you provide a PR or patch, it might be merged.