Expanding the expression, we get:

(3x + 1)(3x - 1) + (3x + 1)(3x + 1)
= 9x^2 - 3x + 3x + 1 + 9x^2 + 3x + 3x + 1
= 9x^2 - 3x + 3x + 1 + 9x^2 + 3x + 3x + 1
= 18x^2 + 6x + 2

So, (3x + 1)(3x - 1) + (3x + 1)(3x + 1) simplifies to 18x^2 + 6x + 2.