I have a vector of strings and I want to return the index of the duplicate values, except for the index of the first occurrence of a duplicate value, given another vector with matches. For example:

```
x <- c("a", "b", "c", "b", "a", "a", "c", "c")
matching_values <- c("a", "b")

```

So I would like to have an integer vector returned with the indexes 4, 5, 6. So the first duplicate of `a` occurs at position 5 and the second duplicate at position 6. The first duplicate for `b` occurs at index 4 and because I did not specify to match `c`, there will be no index returned. Thank you!