diff --git a/app/models/notification/bundle.rb b/app/models/notification/bundle.rb
index 84e1058cb..306b9083f 100644
--- a/app/models/notification/bundle.rb
+++ b/app/models/notification/bundle.rb
@@ -3,10 +3,20 @@ class Notification::Bundle < ApplicationRecord
 
   enum :status, %i[ pending processing delivered ]
 
-  before_create :set_default_window
-
   scope :due, -> { pending.where("ends_at <= ?", Time.current) }
   scope :containing, -> (notification) { where("starts_at <= ? AND ends_at >= ?", notification.created_at, notification.created_at) }
+  scope :overlapping_with, -> (other_bundle) {
+    where(
+      "(starts_at <= ? AND ends_at >= ?) OR (starts_at <= ? AND ends_at >= ?) OR (starts_at >= ? AND ends_at <= ?)",
+      other_bundle.starts_at, other_bundle.starts_at,
+      other_bundle.ends_at, other_bundle.ends_at,
+      other_bundle.starts_at, other_bundle.ends_at
+    )
+  }
+
+  before_create :set_default_window
+
+  validate :validate_no_overlapping
 
   class << self
     def deliver_all
@@ -51,4 +61,14 @@ class Notification::Bundle < ApplicationRecord
     def has_unread_notifications?
       notifications.unread.any?
     end
+
+    def validate_no_overlapping
+      if overlapping_bundles.exists?
+        errors.add(:base, "Bundle window overlaps with an existing pending bundle")
+      end
+    end
+
+    def overlapping_bundles
+      user.notification_bundles.where.not(id: id).overlapping_with(self)
+    end
 end
diff --git a/test/models/notification/bundle_test.rb b/test/models/notification/bundle_test.rb
index 1033a46a7..f9bf4abca 100644
--- a/test/models/notification/bundle_test.rb
+++ b/test/models/notification/bundle_test.rb
@@ -30,6 +30,47 @@ class Notification::BundleTest < ActiveSupport::TestCase
     end
 
     bundle_1, bundle_2 = @user.notification_bundles.last(2)
+    assert_includes bundle_1.notifications, notification_1
+    assert_includes bundle_1.notifications, notification_2
+    assert_includes bundle_2.notifications, notification_3
+  end
 
+  test "overlapping bundles are invalid" do
+    bundle_1 = @user.notification_bundles.create!(
+      starts_at: Time.current,
+      ends_at: 4.hours.from_now,
+      status: :pending
+    )
+
+    bundle_2 = @user.notification_bundles.build(
+      starts_at: 2.hours.from_now,
+      ends_at: 6.hours.from_now,
+      status: :pending
+    )
+    assert_not bundle_2.valid?
+
+    # Bundle with overlapping end time should be invalid
+    bundle_3 = @user.notification_bundles.build(
+      starts_at: 2.hours.ago,
+      ends_at: 2.hours.from_now,
+      status: :pending
+    )
+    assert_not bundle_3.valid?
+
+    # Bundle completely within another bundle should be invalid
+    bundle_4 = @user.notification_bundles.build(
+      starts_at: 1.hour.from_now,
+      ends_at: 3.hours.from_now,
+      status: :pending
+    )
+    assert_not bundle_4.valid?
+
+    # Non-overlapping bundle should be valid
+    bundle_5 = @user.notification_bundles.build(
+      starts_at: 5.hours.from_now,
+      ends_at: 9.hours.from_now,
+      status: :pending
+    )
+    assert bundle_5.valid?
   end
 end