In each of the following 6-digit natural numbers: $333333,225522,118818,707099$, every digit in the number appears at least twice. Find the number of such 6-digit natural numbers.
This is how I'm intending to do.
1) Find the total number of 6-digit combinations.
2) Subtract the number of times there's 0 and 1 repetition of digits so that I can get at least 2 repetitions.
Total Combinations = $9 \times 10^5$
Repeat 0 Times = $9 \times 9 \times 8 \times 7 \times 6 \times 5$
Repeat 1 Time = Not sure how to calculate
The answer is 11754 but I'm struggling to get it!