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