cp: issue correct warning and ignore duplicate source dirs

* src/copy.c (copy_internal): Handle the case where we have the
same destination directory as already encountered, which can only
be due to the corresponding source directory being specified multiple
times.
* tests/cp/duplicate-sources.sh: Add a test for the new multiply
specified directory case, and the existing multiply specified file case.
* tests/local.mk: Reference the new test.
* NEWS: Mention the bug fix.

NEWS

@@ -2,6 +2,12 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Bug fixes
+
+  cp no longer issues an incorrect warning about directory hardlinks when a
+  source directory is specified multiple times.  Instead a warning is issued
+  and the duplicate source directory is skipped, as already the case for files.
+
 ** New features
 
   chroot accepts the new --skip-chdir option to not change the working directory

src/copy.c

@@ -2187,6 +2187,17 @@ copy_internal (char const *src_name, char const *dst_name,
               *copy_into_self = true;
               goto un_backup;
             }
+          else if (same_name (dst_name, earlier_file))
+            {
+              error (0, 0, _("warning: source directory %s "
+                             "specified more than once"),
+                     quote (top_level_src_name));
+              /* We only do backups in move mode and for non dirs,
+                 and in move mode this won't be the issue as the source will
+                 be missing for subsequent attempts.
+                 There we just warn and return here.  */
+              return true;
+            }
           else if (x->dereference == DEREF_ALWAYS)
             {
               /* This happens when e.g., encountering a directory for the

tests/cp/duplicate-sources.sh

@@ -0,0 +1,35 @@
+#!/bin/sh
+# Ensure cp warns about but otherwise ignores source
+# items specified multiple times.
+
+# Copyright (C) 2014 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ cp
+
+mkdir a b || framework_failure_
+touch f || framework_failure_
+
+cp -a a a f f b 2>err || fail=1
+
+cat <<EOF >exp
+cp: warning: source directory 'a' specified more than once
+cp: warning: source file 'f' specified more than once
+EOF
+
+compare exp err || fail=1
+
+Exit $fail

tests/local.mk

@@ -427,6 +427,7 @@ all_tests =					\
   tests/cp/dir-rm-dest.sh			\
   tests/cp/dir-slash.sh				\
   tests/cp/dir-vs-file.sh			\
+  tests/cp/duplicate-sources.sh			\
   tests/cp/existing-perm-dir.sh			\
   tests/cp/existing-perm-race.sh		\
   tests/cp/fail-perm.sh				\