In a formula like X = X + 1, that we can encounter in computer science, X should have the same value on either side of " = ", implying that we would get for, say, X = 5, 5 = 5 + 1, and this seems to result in a contradiction, which would force us to deny the Species-Individuum Structure universally for mathematical formulas.
But in the present case it concerns in fact a prescription, namely that 1 should be added to the value of X.
Hence the formula X = X + 1 means XN + 1 = XN + 1.
Verbally this is : " The new X becomes the old X plus one", implying that we don't have to do with one, but with two different symbols, namely XN + 1 and XN. So no symbol is repeated in this case.