--- a/hgext3rd/topic/__init__.py	Fri Sep 01 18:02:50 2017 +0200
+++ b/hgext3rd/topic/__init__.py	Mon Aug 14 14:36:11 2017 +0530
@@ -460,12 +460,29 @@
     bmstore = repo._bookmarks
     lock = wlock = tr = None
 
+    nodetobook = {}
+    for book, revnode in bmstore.iteritems():
+        if nodetobook.get(revnode):
+            nodetobook[revnode].append(book)
+        else:
+            nodetobook[revnode] = [book]
+
+    # a list of nodes which we have skipped so that we don't print the skip
+    # warning repeatedly
+    skipped = []
+
     if bookmark:
         try:
             node = bmstore[bookmark]
         except KeyError:
             raise error.Abort(_("no such bookmark exists: '%s'") % bookmark)
 
+        if len(nodetobook[node]) > 1:
+            revnum = repo[node].rev()
+            ui.status(_("skipping revision '%d' as it has multiple bookmarks "
+                      "on it\n") % revnum)
+            return
+
         revnum = repo[node].rev()
         try:
             wlock = repo.wlock()
@@ -485,9 +502,17 @@
             lock = repo.lock()
             tr = repo.transaction('debugconvertbookmark')
             for bmark, revnode in sorted(storecopy.iteritems()):
+                revnum = repo[revnode].rev()
+                if revnum in skipped:
+                    continue
+                if len(nodetobook[revnode]) > 1:
+                    ui.status(_("skipping '%d' as it has multiple bookmarks on"
+                              " it\n") % revnum)
+                    skipped.append(revnum)
+                    continue
                 if bmark == '@':
                     continue
-                _convertbmarktopic(ui, repo, repo[revnode].rev(), bmark, tr)
+                _convertbmarktopic(ui, repo, revnum, bmark, tr)
             tr.close()
         finally:
             lockmod.release(tr, lock, wlock)

--- a/tests/test-topic-debugcb.t	Fri Sep 01 18:02:50 2017 +0200
+++ b/tests/test-topic-debugcb.t	Mon Aug 14 14:36:11 2017 +0530
@@ -106,26 +106,17 @@
   |/     topic: hellos
   o  [0:3caf92e45cfb] First commit
   
-XXX: When we have multiple bookmarks on the same changeset, we should skip that
-and do nothing. This should be fixed.
-
   $ hg debugconvertbookmark --all
-  switching to topic book1
-  changed topic to "book1" on 3 revisions
-  changed topic to "book2" on 3 revisions
+  skipping '7' as it has multiple bookmarks on it
   $ hg log -G
-  o  [13:f979f772bd7d] Trying multiple bookmarks
-  |    topic: book2
-  o  [12:2397fdab7b79] Fixing second
-  |    topic: book2
-  o  [11:0dd194861ea1] Fixing first
-  |    topic: book2
-  | @  [10:e738ed1df4b2] Trying multiple bookmarks
-  | |    topic: book1
-  | o  [9:4198fce21412] Fixing second
-  | |    topic: book1
-  | o  [8:221a9ddef504] Fixing first
-  |/     topic: book1
+  @  [7:7c46b4bbdda3] Trying multiple bookmarks
+  |    bookmark: book1
+  |    bookmark:book2
+  |    topic: secondfix
+  o  [6:6efc0524f97a] Fixing second
+  |    topic: secondfix
+  o  [5:0a4244c62a16] Fixing first
+  |    topic: secondfix
   | o  [4:ca8825a7eb18] Second commit
   |/     topic: hellos
   o  [0:3caf92e45cfb] First commit