from collections import defaultdict
from operator import itemgetter
def collect(seq, predicate):
    d = defaultdict(list)
    for item in seq:
        d[predicate(item)].append(item)
    return d

splitted_filenames = [
    ("foo", "123", "txt", "foo_123.txt"),
    ("foo", "456", "dat", "foo_456.dat"),
    ("bar", "789", "py", "bar_789.py")
]

filenames_grouped_by_name = collect(splitted_filenames, itemgetter(0))
for category, filenames in filenames_grouped_by_name.items():
    if len(filenames) > 1:
        print("Found duplicates for filename", category)
        print("Renaming advised for these files:")
        for filename in filenames:
            print(filename[3])
    else:
        print("filename", category, "has no duplicates. No action required.")

#result:
# Found duplicates for filename foo
# Renaming advised for these files:
# foo_123.txt
# foo_456.dat
# filename bar has no duplicates. No action required.