To give you a better understanding of this, consider 5 letters of the alphabet taken 3 at a times with desired repeats. a, b, c, d, e =[5+3 - 1]C3 =7C3 =35 combinations with repeats as follows:
{a, a, a} | {a, a, b} | {a, a, c} | {a, a, d} | {a, a, e} | {a, b, b} | {a, b, c} | {a, b, d} | {a, b, e} | {a, c, c} | {a, c, d} | {a, c, e} | {a, d, d} | {a, d, e} | {a, e, e} | {b, b, b} | {b, b, c} | {b, b, d} | {b, b, e} | {b, c, c} | {b, c, d} | {b, c, e} | {b, d, d} | {b, d, e} | {b, e, e} | {c, c, c} | {c, c, d} | {c, c, e} | {c, d, d} | {c, d, e} | {c, e, e} | {d, d, d} | {d, d, e} | {d, e, e} | {e, e, e} (total: 35) - combinations.