```
a = [1, 2, 9, 5, 1]
b = [9, 8, 7, 6, 5]

```

I want to count the number of duplicates between the two lists. So using the above, I want to return a count of 2 because 9 and 5 are common to both lists.

I tried something like this but it didn't quite work.

```
def filter_(x, y):
    count = 0
    for num in y:
        if num in x:
            count += 1
            return count

```