diff -r 1c21865bf8ba -r 24943df310d4 hgext/evolve.py
--- a/hgext/evolve.py	Tue Jun 12 11:53:02 2012 +0200
+++ b/hgext/evolve.py	Tue Jun 12 13:28:39 2012 +0200
@@ -57,6 +57,11 @@
 #############################
 
 def rewrite(repo, old, updates, head, newbases, commitopts):
+    """Return (nodeid, created) where nodeid is the identifier of the
+    changeset generated by the rewrite process, and created is True if
+    nodeid was actually created. If created is False, nodeid
+    references a changeset existing before the rewrite call.
+    """
     if len(old.parents()) > 1: #XXX remove this unecessary limitation.
         raise error.Abort(_('cannot amend merge changesets'))
     base = old.p1()
@@ -126,29 +131,35 @@
 
         if commitopts.get('edit'):
             new._text = cmdutil.commitforceeditor(repo, new, [])
+        revcount = len(repo)
         newid = repo.commitctx(new)
         new = repo[newid]
-
-        # update the bookmark
-        if bm:
-            repo._bookmarks[bm] = newid
-            bookmarks.write(repo)
+        created = len(repo) != revcount
+        if created:
+            # update the bookmark
+            if bm:
+                repo._bookmarks[bm] = newid
+                bookmarks.write(repo)
 
-        # add evolution metadata
-        repo.addobsolete(new.node(), old.node())
-        for u in updates:
-            repo.addobsolete(u.node(), old.node())
-            repo.addobsolete(new.node(), u.node())
-        oldbookmarks = repo.nodebookmarks(old.node())
-        for book in oldbookmarks:
-            repo._bookmarks[book] = new.node()
-        if oldbookmarks:
-            bookmarks.write(repo)
-
+            # add evolution metadata
+            repo.addobsolete(new.node(), old.node())
+            for u in updates:
+                repo.addobsolete(u.node(), old.node())
+                repo.addobsolete(new.node(), u.node())
+            oldbookmarks = repo.nodebookmarks(old.node())
+            for book in oldbookmarks:
+                repo._bookmarks[book] = new.node()
+            if oldbookmarks:
+                bookmarks.write(repo)
+        else:
+            # newid is an existing revision. It could make sense to
+            # replace revisions with existing ones but probably not by
+            # default.
+            pass
     finally:
         wlock.release()
 
-    return newid
+    return newid, created
 
 def relocate(repo, orig, dest):
     """rewrite <rev> on dest"""
@@ -407,7 +418,11 @@
             def commitfunc(ui, repo, message, match, opts):
                 return repo.commit(message, opts.get('user'), opts.get('date'), match,
                                    editor=e)
-            cmdutil.commit(ui, repo, commitfunc, pats, ciopts)
+            revcount = len(repo)
+            tempid = cmdutil.commit(ui, repo, commitfunc, pats, ciopts)
+            if len(repo) == revcount:
+                # No revision created
+                tempid = None
 
             # find all changesets to be considered updates
             cl = repo.changelog
@@ -429,13 +444,20 @@
             # perform amend
             if opts.get('edit'):
                 opts['force_editor'] = True
-            newid = rewrite(repo, old, updates, head,
-                            [old.p1().node(), old.p2().node()], opts)
-
-            # reroute the working copy parent to the new changeset
-            phases.retractboundary(repo, oldphase, [newid])
-            repo.dirstate.setparents(newid, node.nullid)
-
+            newid, created = rewrite(repo, old, updates, head,
+                                     [old.p1().node(), old.p2().node()], opts)
+            if created:
+                # reroute the working copy parent to the new changeset
+                phases.retractboundary(repo, oldphase, [newid])
+                repo.dirstate.setparents(newid, node.nullid)
+            else:
+                # rewrite() recreated an existing revision, discard
+                # the intermediate revision if any. No need to update
+                # phases or parents.
+                if tempid is not None:
+                    repo.addobsolete(node.nullid, tempid)
+                # XXX: need another message in collapse case.
+                raise error.Abort(_('no updates found'))
         finally:
             wlock.release()
     finally: