Hello, the NES palette brings two repeating colors, one of them will have to be removed.
I’d guess they are there, so that the first row, which contains the mandatory transparent index-0, is filled, so that subsequent rows can have 4 shades of the same color. If you were to remove them, the palette will get all messed up and not as orderly as it is now.
I believe the NES palette is intended to keep the same structure it has on the NES (just rotated and with the 2 rows of black removed), for easier use, which is why that colour is there. Some systems also display these greys slightly differently, so they’re worth distinguishing for some.
The reason there are two blacks is also related to this, rather than the “first index is transparency” thing Aseprite has in indexed mode.
Here’s the NES palette as given by Wikipedia (with the two whites being identical, interestingly), so you can see how the structure correlates. Notice how it’s very different from the palette in Aseprite - this isn’t necessarily because Aseprite is “wrong”, but because the NES wasn’t using RGB and its colours can be interpreted in many different ways in RGB, and Wikipedia and Aseprite use different interpretations, Aseprite’s palette seems to aim more for smooth gradients.
NES enthusiast websites have entire lists of different RGB takes on the NES palette, so if you’re looking for something more “accurate” (for your personal version of what that means), you could load in one of those.