=== modified file 'bzrlib/builtins.py'
--- bzrlib/builtins.py	2010-01-21 17:54:58 +0000
+++ bzrlib/builtins.py	2010-01-21 22:08:58 +0000
@@ -1376,10 +1376,10 @@
 
     _see_also = ['pull', 'working-trees', 'status-flags']
     takes_args = ['dir?']
-    takes_options = ['revision']
+    takes_options = ['revision', 'force']
     aliases = ['up']
 
-    def run(self, dir='.', revision=None):
+    def run(self, dir='.', revision=None, force=False):
         if revision is not None and len(revision) != 1:
             raise errors.BzrCommandError(
                         "bzr update --revision takes exactly one revision")
@@ -1399,6 +1399,26 @@
         branch_location = urlutils.unescape_for_display(branch_location[:-1],
                                                         self.outf.encoding)
         existing_pending_merges = tree.get_parent_ids()[1:]
+        
+        if master:
+            # look for local commits
+            from bzrlib.missing import find_unmerged
+            unmerged_local, _ = find_unmerged(branch, master, restrict='local')
+
+            # look for uncommitted changes
+            changes = tree.changes_from(tree.basis_tree())
+            
+            if not force and changes.has_changed() and unmerged_local:
+                is_lightweight = (tree is not None and tree.bzrdir.root_transport.base != branch.bzrdir.root_transport.base)
+                if is_lightweight:
+                    raise errors.BzrCommandError('The branch that this lightweight checkout is bound to is out of date.'
+                                                 'You should consider updating it first. You can also use --force.')
+                else:
+                    raise errors.BzrCommandError('Cannot update with a'
+                        ' combination of uncommitted changes and local'
+                        ' commits. Please use bzr commit --local to locally'
+                        ' commit your uncommitted changes before updating.')
+
         if master is None:
             old_tip = None
         else:

=== modified file 'bzrlib/tests/blackbox/test_update.py'
--- bzrlib/tests/blackbox/test_update.py	2009-12-23 06:31:19 +0000
+++ bzrlib/tests/blackbox/test_update.py	2010-01-21 22:08:58 +0000
@@ -169,7 +169,7 @@
 
         # now, update checkout ->
         # get all three files and a pending merge.
-        out, err = self.run_bzr('update checkout')
+        out, err = self.run_bzr('update --force checkout')
         self.assertEqual('', out)
         self.assertEqualDiff("""+N  file
 All changes applied successfully.
@@ -315,3 +315,57 @@
 2>All changes applied successfully.
 2>Updated to revision 2 of branch .../master
 ''')
+
+    def _prepare_checkout(self):
+        """Helper function to create a checkout with both local commits
+        and uncommitted changes."""
+        master = self.make_branch_and_tree('master')
+        os.chdir('master')
+        a_file = file('file', 'wt')
+        a_file.write('Foo')
+        a_file.close()
+
+        self.run_bzr('add file')
+        self.run_bzr('commit -m "Commit on Master"')
+
+        # branch
+        self.run_bzr('checkout . ../checkout')
+
+        # add a revision in the checkout
+        os.chdir('../checkout')
+        
+        a_file = file('file', 'wt')
+        a_file.write('Bar')
+        a_file.close()
+        self.run_bzr('commit --local -m "Commit on Branch"')
+        
+        # add local changes
+        a_file = file('file', 'wt')
+        a_file.write('FooBar')
+        a_file.close()
+
+        # bind branch
+        self.run_bzr('bind ../master')
+        
+        os.chdir('..')
+
+    def test_update_checkout_prevent_merge(self):
+        """"Launchpad bug 113809 in bzr "update performs two merges"
+        https://launchpad.net/bugs/113809"""
+
+        self._prepare_checkout()
+
+        os.chdir('checkout')
+        self.expectFailure('Update with a combination of uncommitted changes and local commits causes unexpected merge. (Bug #113809)',
+            self.run_bzr, 'update', retcode=3)
+        self.assertEqual('', out)
+
+    def test_update_checkout_force_merge(self):
+        """"--force should let update work even with local commits
+        and uncommitted changes"""
+
+        self._prepare_checkout()
+
+        os.chdir('checkout')
+        out, err = self.run_bzr('update --force', retcode=1)
+        self.assertEqual('', out)