```python
def pair_off(marks: list[str]) -> list[str]:
    left = []
    for i in range(len(marks)):
        if i > 0 and marks[i] == marks[i - 1]:
            if i + 1 < len(marks) and marks[i + 1] == marks[i]:
                left.pop()
                continue
        left.append(marks[i])
    return left
```