"""Remove accelerator markers (&) from a label, handling escaped ampersands (&&) correctly.

This function processes a string label and removes all single ampersands (&) that are used as accelerator markers. 
Escaped ampersands (&&) are preserved in the output. The function iterates through the label character by character, 
skipping the character immediately following a single ampersand and preserving double ampersands.

Args:
    label (str): The input string containing accelerator markers and possibly escaped ampersands.

Returns:
    str: The label with all single ampersands removed and escaped ampersands preserved.

Examples:
    >>> remove_mnemonic_marker("&a")
    'a'
    >>> remove_mnemonic_marker("a&b&c")
    'abc'
    >>> remove_mnemonic_marker("a&&b")
    'a&&b'
    >>> remove_mnemonic_marker("&a&&b&c")
    'a&&bc'
"""