A B
B C
B D
C E
F G

A B C D E
F G

import sys

res = []  # list of lists
for line in open(sys.argv[1]):
    try:
        x, y = line.split()  # split on space
    except ValueError:
        line = line.rstrip()
        x, y = line.split(',')  # retry with comma
    for l in res:
        if x in l:
            if y not in l:
                l.append(y)
            break
    else:
        res.append([x, y])

for line in res:
    print ' '.join(line)

large big
big great
great vast
small little
little tiny

large big great vast
small little tiny

$ awk -f so-162730.awk -F '[[:space:]]+|,' input.txt
A B C D E
F G