diff -r 4d35bae7caf8 share/roundup/templates/classic/detectors/userauditor.py
--- a/share/roundup/templates/classic/detectors/userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/share/roundup/templates/classic/detectors/userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -73,7 +73,11 @@
             check_main = nodeid not in check_main
 
         # make sure none of the alts are owned by anyone other than us (x!=nodeid)
-        check_alts = [x for x in db.user.filter(None, {'alternate_addresses' : address}) if x != nodeid]
+        check_alts = [
+            x for x in db.user.filter(None, {'alternate_addresses': address})
+            if x != nodeid
+            and address in db.user.get(x, 'alternate_addresses').split('\n')
+        ]
         if check_main or check_alts:
             raise ValueError('Email address %s already in use' % address)
 
diff -r 4d35bae7caf8 share/roundup/templates/devel/detectors/userauditor.py
--- a/share/roundup/templates/devel/detectors/userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/share/roundup/templates/devel/detectors/userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -73,7 +73,11 @@
             check_main = nodeid not in check_main
 
         # make sure none of the alts are owned by anyone other than us (x!=nodeid)
-        check_alts = [x for x in db.user.filter(None, {'alternate_addresses' : address}) if x != nodeid]
+        check_alts = [
+            x for x in db.user.filter(None, {'alternate_addresses': address})
+            if x != nodeid
+            and address in db.user.get(x, 'alternate_addresses').split('\n')
+        ]
         if check_main or check_alts:
             raise ValueError('Email address %s already in use' % address)
 
diff -r 4d35bae7caf8 share/roundup/templates/jinja2/detectors/userauditor.py
--- a/share/roundup/templates/jinja2/detectors/userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/share/roundup/templates/jinja2/detectors/userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -73,7 +73,11 @@
             check_main = nodeid not in check_main
 
         # make sure none of the alts are owned by anyone other than us (x!=nodeid)
-        check_alts = [x for x in db.user.filter(None, {'alternate_addresses' : address}) if x != nodeid]
+        check_alts = [
+            x for x in db.user.filter(None, {'alternate_addresses': address})
+            if x != nodeid
+            and address in db.user.get(x, 'alternate_addresses').split('\n')
+        ]
         if check_main or check_alts:
             raise ValueError('Email address %s already in use' % address)
 
diff -r 4d35bae7caf8 share/roundup/templates/minimal/detectors/userauditor.py
--- a/share/roundup/templates/minimal/detectors/userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/share/roundup/templates/minimal/detectors/userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -73,7 +73,11 @@
             check_main = nodeid not in check_main
 
         # make sure none of the alts are owned by anyone other than us (x!=nodeid)
-        check_alts = [x for x in db.user.filter(None, {'alternate_addresses' : address}) if x != nodeid]
+        check_alts = [
+            x for x in db.user.filter(None, {'alternate_addresses': address})
+            if x != nodeid
+            and address in db.user.get(x, 'alternate_addresses').split('\n')
+        ]
         if check_main or check_alts:
             raise ValueError('Email address %s already in use' % address)
 
diff -r 4d35bae7caf8 share/roundup/templates/responsive/detectors/userauditor.py
--- a/share/roundup/templates/responsive/detectors/userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/share/roundup/templates/responsive/detectors/userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -73,7 +73,11 @@
             check_main = nodeid not in check_main
 
         # make sure none of the alts are owned by anyone other than us (x!=nodeid)
-        check_alts = [x for x in db.user.filter(None, {'alternate_addresses' : address}) if x != nodeid]
+        check_alts = [
+            x for x in db.user.filter(None, {'alternate_addresses': address})
+            if x != nodeid
+            and address in db.user.get(x, 'alternate_addresses').split('\n')
+        ]
         if check_main or check_alts:
             raise ValueError('Email address %s already in use' % address)
 
diff -r 4d35bae7caf8 test/test_userauditor.py
--- a/test/test_userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/test/test_userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -82,6 +82,10 @@
         self.assertRaises(ValueError, self.db.user.set, uid, address='sp_ken@example.com')
         self.assertRaises(ValueError, self.db.user.set, uid, alternate_addresses='kenny@example.com')
 
+        # substrings of existing alternates should be allowed, but not exact matches
+        self.db.user.set(uid, alternate_addresses='p_ken@example.com')
+        self.assertRaises(ValueError, self.db.user.set, uid, alternate_addresses='sp_ken@example.com')
+
     def testBadRoles(self):
         userid = self.db.user.lookup('kyle')
         self.assertRaises(ValueError, self.db.user.set, userid, roles='BadRole')
diff -r 4d35bae7caf8 website/issues/detectors/userauditor.py
--- a/website/issues/detectors/userauditor.py	Sat Aug 29 12:53:43 2026 -0400
+++ b/website/issues/detectors/userauditor.py	Mon Aug 31 10:29:42 2026 +0200
@@ -62,7 +62,11 @@
 
         check_main = db.user.stringFind(address=address)
         # make sure none of the alts are owned by anyone other than us (x!=nodeid)
-        check_alts = [x for x in db.user.filter(None, {'alternate_addresses' : address}) if x != nodeid]
+        check_alts = [
+            x for x in db.user.filter(None, {'alternate_addresses': address})
+            if x != nodeid
+            and address in db.user.get(x, 'alternate_addresses').split('\n')
+        ]
         if check_main or check_alts:
             raise ValueError('Email address %s already in use' % address)