excel has an option called 'conditional formatting' that is up on the main menu. click on it, and there is a drop down called 'highlight cell rules' and then you select 'highlight duplicate values'. i dont know if libre office has this functionality. as an accountant, libre office is shit. i say that as a bit of an open source nerd, too
here's a link to screenshot: https://files.catbox.moe/qm4afh.png
there are other ways mentioned here in this thread, i just thought id point this method out too
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