When saving a revision of a book node, a second revision is created at the same time.

I'm experiencing this too.

I've found that it'll work for users other than user/1, but I don't know which permission makes it work yet... I'll have to do some more testing.

For me, this happens only for Book content: all other content types act normally.

For me, setting the Node module's Administer content permission for a role will cause it to work; but that's not ideal because that permission has security implications.

Ha! I think I found the culprit...

In book.module there is the following code:

/**
 * Implements hook_node_presave().
 */
function book_node_presave($node) {
  // Always save a revision for non-administrators.
  if (!empty($node->book['bid']) && !user_access('administer nodes')) {
    $node->revision = 1;
    // The database schema requires a log message for every revision.
    if (!isset($node->log)) {
      $node->log = '';
    }
  }
  // ...
}

... that's probably what's adding the extra revision.

Wow, okay, adding a revision when non-administrators update book pages has a fascinating history that's almost as old as Drupal itself...

It appears to have been added by Dries on Sun Nov 4, 2001 to prevent non-admins from stomping existing content on Drupal.org.

The line of code to add a revision when a non-administrator saves a book page has been moved around quite a bit, and has changed it's form a number of times, but it has stuck around for a really long time!

It was almost removed in #68418-8: Clean up node submit hooks, but got re-introduced when the commits related to that patch were rolled back in #68418-33: Clean up node submit hooks. That issue eventually got marked "Works as designed" and never went anywhere further.

When the book module was refactored in #146425: improve book module: use nodeapi and menu API, it was changed from a hidden form value in the book content type's node-edit-form's submit function to happening immediately before a book node is saved #146425-181: improve book module: use nodeapi and menu API, because certain actions would force the node's author to change, which was undesirable for drupal.org.

And, the line of code to add revisions is still there in Drupal 8.x.

The following issues may be of interest. They provide a bit more context:

• #4074: Book pages that get updated by non-admins lose their weight and mess up the layout fixed a problem where the weight was being reset when the new revision was being created.
• #5128: book page "administer" removes original author changed it so new revisions would only happen on edits (not when the book was rearranged).
• #6464: Consistency: Cannot disable book revisions fixed the book page edit form so it was clear that a revision would always be saved.
• #86857: wrong log message inserted when using book outliner brought it up, but ended up only changing the log message when book page titles were changed in admin/content/book/{book_parent_nid}.
• #261319: Book page update always creates revision? brought it up, but was marked as fixed when documentation about it was added.

I've filed #2220843: Don't save a revision when book nodes are saved by non-administrators against core.

Leaving this issue open in case it's decided that it shouldn't be backported to D7 core: if that happens, hopefully we can patch Workbench Moderation with a work-around.

Adding a related issue that has a patch and some activity.