--- a/mercurial/cmdutil.py	Mon Feb 24 00:20:47 2020 -0500
+++ b/mercurial/cmdutil.py	Sun Mar 01 19:39:23 2020 +0100
@@ -961,7 +961,7 @@
     raise error.UnknownCommand(cmd, allcmds)
 
 
-def changebranch(ui, repo, revs, label):
+def changebranch(ui, repo, revs, label, opts):
     """ Change the branch name of given revs to label """
 
     with repo.wlock(), repo.lock(), repo.transaction(b'branches'):
@@ -979,7 +979,11 @@
 
         root = repo[roots.first()]
         rpb = {parent.branch() for parent in root.parents()}
-        if label not in rpb and label in repo.branchmap():
+        if (
+            not opts.get(b'force')
+            and label not in rpb
+            and label in repo.branchmap()
+        ):
             raise error.Abort(_(b"a branch of the same name already exists"))
 
         if repo.revs(b'obsolete() and %ld', revs):

--- a/mercurial/commands.py	Mon Feb 24 00:20:47 2020 -0500
+++ b/mercurial/commands.py	Sun Mar 01 19:39:23 2020 +0100
@@ -1338,7 +1338,7 @@
 
             scmutil.checknewlabel(repo, label, b'branch')
             if revs:
-                return cmdutil.changebranch(ui, repo, revs, label)
+                return cmdutil.changebranch(ui, repo, revs, label, opts)
 
             if not opts.get(b'force') and label in repo.branchmap():
                 if label not in [p.branch() for p in repo[None].parents()]:

--- a/tests/test-branch-change.t	Mon Feb 24 00:20:47 2020 -0500
+++ b/tests/test-branch-change.t	Sun Mar 01 19:39:23 2020 +0100
@@ -355,6 +355,13 @@
   abort: a branch of the same name already exists
   [255]
 
+  $ hg branch -r . stable --force
+  changed branch on 1 changesets
+  $ hg branches
+  stable                        34:d1c2addda4a2
+  jkl                           29:6bc1c6c2c9da (inactive)
+  ghi                           28:2f1019bd29d2 (inactive)
+
 Changing branch on public changeset
 
   $ hg phase -r . -p