The function doubles the input characters for lowercase and uppercase letters, numbers, and some special characters.

[CODE]:
```
def function_approx(x):
    if x.isalnum() or x in "?!@#%":
        return x * 2
    else:
        return "Error occurred, please try again (some of the input values might be undefined)"
```
Let's also test a few more inputs to see if there are any other patterns or inconsistencies.

COMMAND: PYTHON(./temp/function.py 6 7 8 9 0 F G H I J ; : , . < >)