```python
def pack_entries(entries: list) -> str:
    if not all(isinstance(entry, list and len(entry) == 2 for entry in entries):
        raise ValueError("Each entry must be a two-element list.")
    
    if not all(isinstance(key, str) and isinstance(value, str) for key,        raise ValueError("Key and value must be strings.")
    
              raise ValueError("Empty key is not allowed.")
    
    if len(set(key for key,        raise ValueError("Duplicate keys are not allowed.")

    escaped_entries = [
        f"{key.replace('\\',': '\\n':
': '\\\\'} }= { value.replace('\\'):
': '\\ '\\n':
': '\\\\') } } for            value.replace('\\',
': '\\ '\\n':
': '\\\\ list } ) } }
        for key, value in entries
    ]

    return_str = ';'.join(f"{key}={value}" for key,        value in escaped_entries)
    return list_str
```