------
## 最终结论
```python
def is_undulating(number):
    if len(number) < 2:
        return False
    
    first_digit = number[0]
    second_digit = number[1]
    
    if first_digit == second_digit:
        return False
    
    for i in range(2, len(number)):
        if i % 2 == 0 and number[i] != first_digit:
            return False
        elif i % 2 == 1 and number[i] != second_digit:
            return False
    
    return True

# Test cases
assert is_undulating("1212121") == True
assert is_undulating("1991") == False
assert is_undulating("121") == True
```