Suppose that the symbols you have are 0235x8y. How many combinations can you form with these 7 symbols? Suppose that you have a list of all of them. Two such combinations would be 253x0y8 and 2x350y8. But x is 5 so these supposedly two combinations, 253x0y8 and 2x350y8, are actually the same one, 25350y8. In fact, in your list of all combinations of the symbols 0235x8y every one has a twin, just interchange the 5 and the x. Hence your list is twice as long as it should be.
Now apply the same argument to the 8 and the y.