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Built-in roms are located here: C:\Program Files\WindowsApps\50446Nesbox.16735B73F29AD_1.1.1.0_x64__8y3dwps6jawp4\Assets\Roms

I noticed that the emulator also makes a second (redundant) copy of the roms you load inside one of the folders I mentioned above, along with the save state (if you make one) and original SRAM save of the game, not using the original location of the ROM. No need to duplicate it, this is something that should be fixed, adding paths to roms you already have located somewhere else instead.