diff --git a/hr_shift/models/shift_planning.py b/hr_shift/models/shift_planning.py
index 80f2352..ca12784 100644
--- a/hr_shift/models/shift_planning.py
+++ b/hr_shift/models/shift_planning.py
@@ -380,6 +380,42 @@ def _constrain_template_id(self):
                     _("This employee is on leave so can't be assigned to this shift")
                 )
 
+    @api.constrains("start_time", "end_time", "employee_id", "state")
+    def _constrain_line_overlap(self):
+        lines = self.filtered(
+            lambda line: (
+                line.state == "assigned"
+                and line.start_time
+                and line.end_time
+                and line.employee_id
+            )
+        )
+        if not lines:
+            return
+        others = self.search(
+            [
+                ("id", "not in", lines.ids),
+                ("state", "=", "assigned"),
+                ("employee_id", "in", lines.employee_id.ids),
+                ("start_time", "<", max(lines.mapped("end_time"))),
+                ("end_time", ">", min(lines.mapped("start_time"))),
+            ]
+        )
+        for line in lines:
+            if others.filtered_domain(
+                [
+                    ("employee_id", "=", line.employee_id.id),
+                    ("start_time", "<", line.end_time),
+                    ("end_time", ">", line.start_time),
+                ]
+            ):
+                raise UserError(
+                    _(
+                        "%(employee)s already has an overlapping shift on this period.",
+                        employee=line.employee_id.name,
+                    )
+                )
+
     @api.depends("template_id")
     def _compute_state(self):
         for shift in self:
diff --git a/hr_shift/tests/test_hr_shift.py b/hr_shift/tests/test_hr_shift.py
index ae4fa2a..ebc3824 100644
--- a/hr_shift/tests/test_hr_shift.py
+++ b/hr_shift/tests/test_hr_shift.py
@@ -5,6 +5,7 @@
 import pytz
 
 from odoo import fields
+from odoo.exceptions import UserError
 from odoo.tests import Form
 from odoo.tools import mute_logger
 
@@ -155,3 +156,36 @@ def test_hr_shift_planning_full(self):
         shift_b_line_1 = shift_b.line_ids.filtered(lambda x: x.day_number == "1")
         self.assertEqual(shift_b_line_1.state, "assigned")
         self.assertEqual(shift_b_line_1.template_id, self.template_afternoon)
+
+    def test_hr_shift_planning_line_overlap_is_refused(self):
+        self.planning.generate_shifts()
+        shift = self.planning.shift_ids.filtered(
+            lambda s: s.employee_id == self.employee_a
+        )
+        line = shift.line_ids.filtered(lambda x: x.day_number == "0")
+        line.template_id = self.template_morning
+        with self.assertRaises(UserError):
+            self.env["hr.shift.planning.line"].create(
+                {
+                    "shift_id": shift.id,
+                    "day_number": "0",
+                    "template_id": self.template_morning.id,
+                }
+            )
+
+    def test_hr_shift_planning_line_touch_to_touch_is_allowed(self):
+        self.planning.generate_shifts()
+        shift = self.planning.shift_ids.filtered(
+            lambda s: s.employee_id == self.employee_a
+        )
+        line = shift.line_ids.filtered(lambda x: x.day_number == "0")
+        line.template_id = self.template_morning
+        extra = self.env["hr.shift.planning.line"].create(
+            {
+                "shift_id": shift.id,
+                "day_number": "0",
+                "template_id": self.template_afternoon.id,
+            }
+        )
+        self.assertEqual(extra.state, "assigned")
+        self.assertEqual(line.end_time, extra.start_time)