The function takes a string input and performs the following transformations:
- It replaces every 'l' in the string with 'o'.
- It replaces every 'd' in the string with 'u'.
- It duplicates the last character of the string.

[CODE]:
```python
def f(x):
    x = x.replace('l', 'o')
    x = x.replace('d', 'u')
    x += x[-1]
    return x
```

This Python code approximates the behavior of the function by applying the transformations described above.