diff --git a/addons/account/i18n/account.pot b/addons/account/i18n/account.pot
index f02dee6340f2c..91346b0d6a500 100644
--- a/addons/account/i18n/account.pot
+++ b/addons/account/i18n/account.pot
@@ -688,7 +688,6 @@ msgstr ""
 #: code:addons/account/models/res_users.py:22
 #, python-format
 msgid "A user cannot have both Tax B2B and Tax B2C.\n"
-"Problematic user(s): %s\n"
 "You should go in General Settings, and choose to display Product Prices\n"
 "either in 'Tax-Included' or in 'Tax-Excluded' mode\n"
 "(or switch twice the mode if you are already in the desired one)."
diff --git a/addons/account/models/res_users.py b/addons/account/models/res_users.py
index 8030423e74d3b..013acde54ffbe 100644
--- a/addons/account/models/res_users.py
+++ b/addons/account/models/res_users.py
@@ -13,14 +13,11 @@ class Users(models.Model):
     def _check_one_user_type(self):
         super(Users, self)._check_one_user_type()
 
-        users_with_both_groups = self.filtered(lambda user:
-            user.has_group('account.group_show_line_subtotals_tax_included') and
-            user.has_group('account.group_show_line_subtotals_tax_excluded')
-        )
-        if users_with_both_groups:
-            names = ", ".join(users_with_both_groups.mapped('name'))
+        g1 = self.env.ref('account.group_show_line_subtotals_tax_included')
+        g2 = self.env.ref('account.group_show_line_subtotals_tax_excluded')
+
+        if self._has_multiple_groups([g1.id, g2.id]):
             raise ValidationError(_("A user cannot have both Tax B2B and Tax B2C.\n"
-                                    "Problematic user(s): %s\n"
                                     "You should go in General Settings, and choose to display Product Prices\n"
                                     "either in 'Tax-Included' or in 'Tax-Excluded' mode\n"
-                                    "(or switch twice the mode if you are already in the desired one).") % names)
+                                    "(or switch twice the mode if you are already in the desired one)."))
diff --git a/odoo/addons/base/models/res_users.py b/odoo/addons/base/models/res_users.py
index b524a6770ea11..002693ee7815f 100644
--- a/odoo/addons/base/models/res_users.py
+++ b/odoo/addons/base/models/res_users.py
@@ -409,10 +409,43 @@ def _check_action_id(self):
     @api.multi
     @api.constrains('groups_id')
     def _check_one_user_type(self):
-        for user in self:
-            if len(user.groups_id.filtered(lambda x: x.category_id.xml_id == 'base.module_category_user_type')) > 1:
+        """We check that no users are both portal and users (same with public).
+           This could typically happen because of implied groups.
+        """
+        user_types_category = self.env.ref('base.module_category_user_type', raise_if_not_found=False)
+        user_types_groups = self.env['res.groups'].search(
+            [('category_id', '=', user_types_category.id)]) if user_types_category else False
+        if user_types_groups:  # needed at install
+            if self._has_multiple_groups(user_types_groups.ids):
                 raise ValidationError(_('The user cannot have more than one user types.'))
 
+    @api.multi
+    def _has_multiple_groups(self, group_ids):
+        """The method is not fast if the list of ids is very long;
+           so we rather check all users than limit to the size of the group
+        :param group_ids: list of group ids
+        :return: boolean: is there at least a user in at least 2 of the provided groups
+        """
+        if group_ids:
+            args = [tuple(group_ids)]
+            if len(self.ids) == 1:
+                where_clause = "AND r.uid = %s"
+                args.append(self.id)
+            else:
+                where_clause = ""  # default; we check ALL users (actually pretty efficient)
+            query = """
+                    SELECT 1 FROM res_groups_users_rel WHERE EXISTS(
+                        SELECT r.uid
+                        FROM res_groups_users_rel r
+                        WHERE r.gid IN %s""" + where_clause + """
+                        GROUP BY r.uid HAVING COUNT(r.gid) > 1
+                    )
+            """
+            self.env.cr.execute(query, args)
+            return bool(self.env.cr.fetchall())
+        else:
+            return False
+
     @api.multi
     def toggle_active(self):
         for user in self:
@@ -955,6 +988,7 @@ def write(self, values):
                            JOIN group_imply i ON (r.gid = i.hid)
                           WHERE i.gid = %(gid)s
                 """, dict(gid=group.id))
+            self._check_one_user_type()
         return res
 
 class UsersImplied(models.Model):
diff --git a/odoo/addons/base/tests/test_user_has_group.py b/odoo/addons/base/tests/test_user_has_group.py
index 0151bd2354c64..98e699a071841 100644
--- a/odoo/addons/base/tests/test_user_has_group.py
+++ b/odoo/addons/base/tests/test_user_has_group.py
@@ -143,3 +143,21 @@ def test_two_user_types(self):
              })
         with self.assertRaises(ValidationError):
             self.grp_internal.users = [(4, test_user.id)]
+
+    def test_two_user_types_implied_groups(self):
+        """Contrarily to test_two_user_types, we simply add an implied_id to a group.
+           This will trigger the addition of the relevant users to the relevant groups;
+           if, say, this was done in SQL and thus bypassing the ORM, it would bypass the constraints
+           and thus give us a case uncovered by the aforementioned test.
+        """
+        grp_test = self.env["res.groups"].create(
+            {"name": "test", "implied_ids": [(6, 0, [self.grp_internal.id])]})
+
+        test_user = self.env['res.users'].create({
+            'login': 'test_user_portal',
+            'name': "Test User with one user types",
+            'groups_id': [(6, 0, [grp_test.id])]
+        })
+
+        with self.assertRaises(ValidationError):
+            grp_test.write({'implied_ids': [(4, self.grp_portal.id)]})