------
## 最终结论
```python
def is_sorted(lst):
    # Check if the list is sorted
    for i in range(len(lst) - 1):
        if lst[i] > lst[i + 1]:
            return False
    
    # Check for duplicate elements
    count = {}
    for num in lst:
        if num in count:
            count[num] += 1
            if count[num] > 2:  # If any number appears more than twice
                return False
        else:
            count[num] = 1
            
    return True
```