The function f(x) replaces the second character in each input string with the letter 'a', unless the second character is already 'a'. In that case, it remains unchanged.

[CODE]:
```python
def f(x):
    output = ""
    for string in x:
        if len(string) > 1:
            if string[1] == 'a':
                output += string
            else:
                output += string[0] + 'a' + string[2:]
        else:
            output += string
    return output
```

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