diff --git a/HISTORY.rst b/HISTORY.rst
index 3612297..d3386f7 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -1,6 +1,9 @@
 Changelog
 ---------
 
+- Reservations added to the same session may not be duplicated anymore.
+  [href]
+
 - Errors raised during reservation now have a 'reservation' attribute.
   [href]
 
diff --git a/libres/db/scheduler.py b/libres/db/scheduler.py
index 1998a3d..4dc9cbf 100644
--- a/libres/db/scheduler.py
+++ b/libres/db/scheduler.py
@@ -985,6 +985,19 @@ def new_reservations_by_dates(dates):
         if not reservations:
             raise errors.InvalidReservationError
 
+        # have a very simple overlap check for reservations, it's not important
+        # that this catches *all* possible problems - that's being handled
+        # by the reservation slots - but it should stop us from adding the same
+        # reservation twice on a single session
+        if session_id:
+            found = self.queries.reservations_by_session(session_id)
+            found = found.with_entities(Reservation.target, Reservation.start)
+            found = set(found.all())
+
+            for reservation in reservations:
+                if (reservation.target, reservation.start) in found:
+                    raise errors.OverlappingReservationError
+
         for reservation in reservations:
             self.session.add(reservation)
 
diff --git a/libres/modules/errors.py b/libres/modules/errors.py
index 207642b..bfbd875 100644
--- a/libres/modules/errors.py
+++ b/libres/modules/errors.py
@@ -90,6 +90,10 @@ def __init__(self, start, end, existing):
         self.existing = existing
 
 
+class OverlappingReservationError(LibresError):
+    pass
+
+
 class AffectedReservationError(LibresError):
 
     def __init__(self, existing):
diff --git a/libres/tests/test_reservation.py b/libres/tests/test_reservation.py
index a3854e8..666cb1c 100644
--- a/libres/tests/test_reservation.py
+++ b/libres/tests/test_reservation.py
@@ -2,7 +2,9 @@
 
 from datetime import datetime, timedelta
 from libres.db.models import Reservation
+from libres.modules.errors import OverlappingReservationError
 from sedate import standardize_date
+from uuid import uuid4
 
 
 def test_reservation_title():
@@ -53,3 +55,36 @@ def test_group_reservation_timespans(scheduler):
     assert timespans[1].start == standardize_date(dates[1][0], 'Europe/Zurich')
     assert timespans[1].end == standardize_date(dates[1][1], 'Europe/Zurich')\
         - timedelta(microseconds=1)
+
+
+def test_overlapping_reservations(scheduler):
+    start = datetime(2015, 2, 5, 15)
+    end = datetime(2015, 2, 5, 16)
+
+    scheduler.allocate(dates=(start, end))
+
+    # overlapping reservations are only prohibited on a per-session base
+    scheduler.reserve(
+        email='test@example.org',
+        dates=(start, end),
+        session_id=uuid4()
+    )
+    scheduler.reserve(
+        email='test@example.org',
+        dates=(start, end),
+        session_id=uuid4()
+    )
+
+    session_id = uuid4()
+    scheduler.reserve(
+        email='test@example.org',
+        dates=(start, end),
+        session_id=session_id
+    )
+
+    with pytest.raises(OverlappingReservationError):
+        scheduler.reserve(
+            email='test@example.org',
+            dates=(start, end),
+            session_id=session_id
+        )
diff --git a/libres/tests/test_scheduler.py b/libres/tests/test_scheduler.py
index 3e8cba5..bbe57fe 100644
--- a/libres/tests/test_scheduler.py
+++ b/libres/tests/test_scheduler.py
@@ -366,21 +366,24 @@ def test_reserve_single_token_per_session(scheduler):
 
     session_id = new_uuid()
 
-    start = datetime(2011, 1, 1, 15)
-    end = datetime(2011, 1, 1, 16)
-
-    scheduler.allocate((start, end), quota=3)
+    a1, a2 = scheduler.allocate(
+        dates=(
+            datetime(2011, 1, 1, 15), datetime(2011, 1, 1, 16),
+            datetime(2011, 1, 2, 15), datetime(2011, 1, 2, 16)
+        ),
+        quota=1
+    )
 
     token1 = scheduler.reserve(
         email=u'test@example.org',
-        dates=(start, end),
+        dates=(a1.start, a1.end),
         session_id=session_id,
         single_token_per_session=True
     )
 
     token2 = scheduler.reserve(
         email=u'test@example.org',
-        dates=(start, end),
+        dates=(a2.start, a2.end),
         session_id=session_id,
         single_token_per_session=True
     )
@@ -391,14 +394,14 @@ def test_reserve_single_token_per_session(scheduler):
 
     token3 = scheduler.reserve(
         email=u'test@example.org',
-        dates=(start, end),
+        dates=(a1.start, a1.end),
         session_id=session_id,
         single_token_per_session=True
     )
 
     token4 = scheduler.reserve(
         email=u'test@example.org',
-        dates=(start, end),
+        dates=(a2.start, a2.end),
         session_id=session_id,
         single_token_per_session=True
     )