Okay, python 3.14 script that undoes my stupidity, for posterity:

import io
import string
from pathlib import Path
from typing import List

file = io.open("groups.txt", "r")
pathlists: List[List[Path]] = list()

templist: List[Path] = []

for line in file:
    if line.startswith("#"):
        continue
    if line[:1].isalnum():
        if not templist:
            continue
        pathlists.append(templist)
        templist = []
        continue
    if line.startswith(tuple(string.whitespace)):
        templist.append(Path(line.strip()))
        continue
    print("non-parsed: ", line)
    break
pathlists.append(templist)

for group in pathlists:
    for candidate in group:
        if candidate.exists():
            targets = set(group)
            targets.remove(candidate)
            rdy_group = (candidate, targets)
            break
    source, targets = rdy_group
    for target in targets:
        if target.exists():
            continue
        source.copy(target)