Gracefully handle duplicate symlinks when extracting

If the archive being extracted contains multiple copies
of the same symlink, extract only the first of them and
skip the rest.  The use case is described in

https://lists.gnu.org/archive/html/bug-tar/2021-01/msg00026.html

The bug was introduced by 2d3396c3ea.

* src/extract.c (create_placeholder_file): If placeholder already
exists and is registered in the delayed_link list, return immediately.

src/extract.c

@@ -1323,6 +1323,41 @@ extract_file (char *file_name, int typeflag)
   return status;
 }
 
+/* Find a delayed_link structure corresponding to the source NAME.
+   Such a structure exists in the delayed link list only if the link
+   placeholder file has been created. Therefore, try to stat the NAME
+   first. If it doesn't exist, there is no matching entry in the list.
+   Otherwise, look for the entry in list which has the matching dev
+   and ino numbers.
+   
+   This approach avoids scanning the singly-linked list in obvious cases
+   and does not rely on comparing file names, which may differ for
+   various reasons (e.g. relative vs. absolute file names).
+ */
+static struct delayed_link *
+find_delayed_link_source (char const *name)
+{
+  struct delayed_link *dl;
+  struct stat st;
+
+  if (!delayed_link_head)
+    return NULL;
+  
+  if (fstatat (chdir_fd, name, &st, AT_SYMLINK_NOFOLLOW))
+    {
+      if (errno != ENOENT)
+	stat_error (name);
+      return NULL;
+    }
+  
+  for (dl = delayed_link_head; dl; dl = dl->next)
+    {
+      if (dl->dev == st.st_dev && dl->ino == st.st_ino)
+	break;
+    }
+  return dl;
+}
+  
 /* Create a placeholder file with name FILE_NAME, which will be
    replaced after other extraction is done by a symbolic link if
    IS_SYMLINK is true, and by a hard link otherwise.  Set
@@ -1342,6 +1377,15 @@ create_placeholder_file (char *file_name, bool is_symlink, bool *interdir_made,
 
   while ((fd = openat (chdir_fd, file_name, O_WRONLY | O_CREAT | O_EXCL, 0)) < 0)
     {
+      if (errno == EEXIST && find_delayed_link_source (file_name))
+	{
+	  /* The placeholder file has already been created.  This means
+	     that the link being extracted is a duplicate of an already
+	     processed one.  Skip it.
+	   */
+	  return 0;
+	}
+      
       switch (maybe_recoverable (file_name, false, interdir_made))
 	{
 	case RECOVER_OK:
@@ -1416,41 +1460,6 @@ create_placeholder_file (char *file_name, bool is_symlink, bool *interdir_made,
   return -1;
 }
 
-/* Find a delayed_link structure corresponding to the source NAME.
-   Such a structure exists in the delayed link list only if the link
-   placeholder file has been created. Therefore, try to stat the NAME
-   first. If it doesn't exist, there is no matching entry in the list.
-   Otherwise, look for the entry in list which has the matching dev
-   and ino numbers.
-   
-   This approach avoids scanning the singly-linked list in obvious cases
-   and does not rely on comparing file names, which may differ for
-   various reasons (e.g. relative vs. absolute file names).
- */
-static struct delayed_link *
-find_delayed_link_source (char const *name)
-{
-  struct delayed_link *dl;
-  struct stat st;
-
-  if (!delayed_link_head)
-    return NULL;
-  
-  if (fstatat (chdir_fd, name, &st, AT_SYMLINK_NOFOLLOW))
-    {
-      if (errno != ENOENT)
-	stat_error (name);
-      return NULL;
-    }
-  
-  for (dl = delayed_link_head; dl; dl = dl->next)
-    {
-      if (dl->dev == st.st_dev && dl->ino == st.st_ino)
-	break;
-    }
-  return dl;
-}
-  
 static int
 extract_link (char *file_name, int typeflag)
 {