diff --git a/forwardport/models/project.py b/forwardport/models/project.py
index fd9b910d..b1ac79d4 100644
--- a/forwardport/models/project.py
+++ b/forwardport/models/project.py
@@ -538,10 +538,11 @@ class PullRequests(models.Model):
         if not self:
             return
 
-        ref = self[0]
+        all_sources = [(p.source_id or p._get_root()) for p in self]
+        all_targets = [s._find_next_target(p) for s, p in zip(all_sources, self)]
 
-        base = ref.source_id or ref._get_root()
-        all_targets = [(p.source_id or p._get_root())._find_next_target(p) for p in self]
+        ref = self[0]
+        base = all_sources[0]
         target = all_targets[0]
         if target is None:
             _logger.info(
@@ -550,6 +551,13 @@ class PullRequests(models.Model):
             )
             return  # QUESTION: do the prs need to be updated?
 
+        # check if the PRs have already been forward-ported: is there a PR
+        # with the same source targeting the next branch in the series
+        for source in all_sources:
+            if self.search_count([('source_id', '=', source.id), ('target', '=', target.id)]):
+                _logger.info("Will not forward-port %s: already ported", ref.display_name)
+                return
+
         # check if all PRs in the batch have the same "next target" , bail if
         # that's not the case as it doesn't make sense for forward one PR from
         # a to b and a linked pr from a to c
diff --git a/forwardport/tests/test_updates.py b/forwardport/tests/test_updates.py
index 104e9f1f..bea30fa8 100644
--- a/forwardport/tests/test_updates.py
+++ b/forwardport/tests/test_updates.py
@@ -2,6 +2,9 @@
 Test cases for updating PRs during after the forward-porting process after the
 initial merge has succeeded (and forward-porting has started)
 """
+import sys
+
+import pytest
 
 from utils import seen, re_matches, Commit, make_basic
 
@@ -260,3 +263,105 @@ More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port
 
 You may want or need to manually update any followup PR.""" % pr1_id.display_name)
     ]
+
+def test_duplicate_fw(env, make_repo, setreviewers, config, users):
+    """ Test for #451
+    """
+    # 0 - 1 - 2 - 3 - 4  master
+    #             \ - 31 v3
+    #         \ - 21     v2
+    #     \ - 11         v1
+    repo = make_repo('proj')
+    with repo:
+        _, c1, c2, c3, _ = repo.make_commits(
+            None,
+            Commit('0', tree={'f': 'a'}),
+            Commit('1', tree={'f': 'b'}),
+            Commit('2', tree={'f': 'c'}),
+            Commit('3', tree={'f': 'd'}),
+            Commit('4', tree={'f': 'e'}),
+            ref='heads/master'
+        )
+        repo.make_commits(c1, Commit('11', tree={'g': 'a'}), ref='heads/v1')
+        repo.make_commits(c2, Commit('21', tree={'h': 'a'}), ref='heads/v2')
+        repo.make_commits(c3, Commit('31', tree={'i': 'a'}), ref='heads/v3')
+
+    proj = env['runbot_merge.project'].create({
+        'name': 'a project',
+        'github_token': config['github']['token'],
+        'github_prefix': 'hansen',
+        'fp_github_token': config['github']['token'],
+        'branch_ids': [
+            (0, 0, {'name': 'master', 'fp_sequence': 0, 'fp_target': True}),
+            (0, 0, {'name': 'v3', 'fp_sequence': 1, 'fp_target': True}),
+            (0, 0, {'name': 'v2', 'fp_sequence': 2, 'fp_target': True}),
+            (0, 0, {'name': 'v1', 'fp_sequence': 3, 'fp_target': True}),
+        ],
+        'repo_ids': [
+            (0, 0, {
+                'name': repo.name,
+                'required_statuses': 'ci',
+                'fp_remote_target': repo.name,
+            })
+        ]
+    })
+    setreviewers(*proj.repo_ids)
+
+    # create a PR in v1, merge it, then create all 3 ports
+    with repo:
+        repo.make_commits('v1', Commit('c0', tree={'z': 'a'}), ref='heads/hugechange')
+        prv1 = repo.make_pr(target='v1', head='hugechange')
+        repo.post_status('hugechange', 'success', 'ci')
+        prv1.post_comment('hansen r+', config['role_reviewer']['token'])
+    env.run_crons()
+    PRs = env['runbot_merge.pull_requests']
+    prv1_id = PRs.search([
+        ('repository.name', '=', repo.name),
+        ('number', '=', prv1.number),
+    ])
+    assert prv1_id.state == 'ready'
+    with repo:
+        repo.post_status('staging.v1', 'success', 'ci')
+    env.run_crons()
+    assert prv1_id.state == 'merged'
+
+    parent = prv1_id
+    while True:
+        child = PRs.search([('parent_id', '=', parent.id)])
+        if not child:
+            break
+
+        assert child.state == 'opened'
+        with repo:
+            repo.post_status(child.head, 'success', 'ci')
+        env.run_crons()
+        parent = child
+    pr_ids = _, prv2_id, prv3_id, prmaster_id = PRs.search([], order='number')
+    _, prv2, prv3, prmaster = [repo.get_pr(p.number) for p in pr_ids]
+    assert pr_ids.mapped('target.name') == ['v1', 'v2', 'v3', 'master']
+    assert pr_ids.mapped('state') == ['merged', 'validated', 'validated', 'validated']
+    assert repo.read_tree(repo.commit(prmaster_id.head)) == {'f': 'e', 'z': 'a'}
+
+    with repo:
+        repo.make_commits('v2', Commit('c0', tree={'z': 'b'}), ref=prv2.ref, make=False)
+    env.run_crons()
+    assert pr_ids.mapped('state') == ['merged', 'opened', 'validated', 'validated']
+    assert repo.read_tree(repo.commit(prv2_id.head)) == {'f': 'c', 'h': 'a', 'z': 'b'}
+    assert repo.read_tree(repo.commit(prv3_id.head)) == {'f': 'd', 'i': 'a', 'z': 'b'}
+    assert repo.read_tree(repo.commit(prmaster_id.head)) == {'f': 'e', 'z': 'b'}
+
+    assert prv2_id.source_id == prv1_id
+    assert not prv2_id.parent_id
+
+    env.run_crons()
+    assert PRs.search([], order='number') == pr_ids
+
+    with repo:
+        repo.post_status(prv2.head, 'success', 'ci')
+        prv2.post_comment('hansen r+', config['role_reviewer']['token'])
+    env.run_crons()
+    with repo:
+        repo.post_status('staging.v2', 'success', 'ci')
+    env.run_crons()
+    # env.run_crons()
+    assert PRs.search([], order='number') == pr_ids