Test case with uutils' du command:

$ mkdir test
$ cd test
$ mkdir dir1
$ dd if=/dev/urandom of=file1 bs=1M count=3
$ dd if=/dev/urandom of=dir1/file1 bs=1M count=5
$ mkdir dir2
$ dd if=/dev/urandom of=dir2/file1 bs=1M count=7
$ du -smc dir1 .
5	dir1
15	.
20	total
$ du --version
du (uutils coreutils) 0.2.2

The same test with GNU coreutils' du command:

$ du -smc dir1 .
5	dir1
10	.
15	total
$ du --version
du (GNU coreutils) 9.5
Copyright (C) 2024 Free Software Foundation, Inc

The correct output is the one provided by GNU coreutils, because the test directory contains a total of 15 MiB data and its subdirectory dir1 contains 5 MiB. During an invocation, du should not count files more than once, even when the arguments passed to it specify so.

This is a incompatibility bug of uutils wrt GNU coreutils and it should be treated according to the Goals:

Our key objectives include:
Matching GNU's output (stdout and error code) exactly

Please check the relevant discussion at AskUbuntu.com too.