import re


def curlify(content: str) -> str:
    """
    Adds a pair of curly braces to the content. If the content already contains curly brackets, nothing will happen.

    Example:
    - curlify("Hello") returns "{Hello}".
    - curlify("{Hello}") returns "{Hello}".
    """
    curlify_pattern = re.compile(r'^{.*}$', re.DOTALL)
    # Check if curly brackets are already set
    if not re.fullmatch(curlify_pattern, content):
        # Add extra curly brackets to title. Preserves lowercase/uppercase in BIBTeX
        content = "{{{}}}".format(content)
    return content