```python
def fill_between(entries: list, filler: str) -> list:
    """
    Returns a list with a filler placed between every two neighboring entries.
    Three entries come back as five. A list of one entry, or none at all,
    takes no filler.
    """
    if not entries:
        return []

    result = []
    for i in range(len(entries) - 1):
        result.extend([entries[i], filler])
    result.append(entries[-1])

    return result
```