After you get all of the "you're so wrong" out of your system, hear me out. Lower case and capital letters are completely distinct. Even though they are considered the same letter, take the same name, and make the sound, the fact remains that they are totally different letters.
It may not be easy to see this, but the capital and lower case "A" look nothing alike. There is no reason to think that they have the same sound and meaning. There are some exceptions, like "O" and "X", because they are just different sizes of the same shape, but we do recognize them as unique characters. The capital and lower case versions are different sizes and are used in different ways. They are still distinct.
So that's 52 letters you know. So how come I said 104? That's because all of those letters get doubled with cursive.
I know that script handwriting is fallen out of favor (and to be candid, I'm perfectly fine with that), but I believe people can still read it for the most part, even if they don't write it. And that is how you know 104 letters.
So, if you ever find yourself concerned or hesitant or making excuses to not learn a new alphabet, remind yourself that you already know 4 of them, so what's the harm in learning one more?