diff --git a/app/models/transfer/state.rb b/app/models/transfer/state.rb
index 082eb52dc1..51fc76fc99 100644
--- a/app/models/transfer/state.rb
+++ b/app/models/transfer/state.rb
@@ -49,7 +49,7 @@ def self.included(base) # rubocop:todo Metrics/MethodLength
                     query_conditions << ' OR (transfer_requests.state IS NULL AND plate_purposes.stock_plate=TRUE)'
                   end
 
-                  joins(:transfer_requests_as_target, :plate_purpose).where([query_conditions, states])
+                  joins(:transfer_requests_as_target, :plate_purpose).where([query_conditions, states]).distinct
                 else
                   all
                 end
@@ -59,6 +59,7 @@ def self.included(base) # rubocop:todo Metrics/MethodLength
   end
 
   # Tube specific behaviour
+  # TODO: Seems to be broken due to reference to (non-existent) assets table, see Y26-155.
   module TubeState
     def self.included(base) # rubocop:todo Metrics/MethodLength
       base.class_eval do
diff --git a/spec/models/transfer/plate_state_spec.rb b/spec/models/transfer/plate_state_spec.rb
new file mode 100644
index 0000000000..c2f400b00d
--- /dev/null
+++ b/spec/models/transfer/plate_state_spec.rb
@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Transfer::State::PlateState do
+  describe '.in_state' do
+    # Three pairs of parent/child plates are created, with transfer requests flowing
+    # from the parent wells into the child wells. Each pair has a distinct transfer
+    # request state so that the in_state scope can be exercised in isolation.
+    let(:parent_of_passed_plate) { create(:plate, well_count: 2) }
+    let(:parent_of_failed_plate) { create(:plate, well_count: 2) }
+    let(:parent_of_pending_plate) { create(:plate, well_count: 2) }
+
+    let!(:passed_plate) { create(:plate_with_empty_wells, well_count: 2) }
+    let!(:failed_plate) { create(:plate_with_empty_wells, well_count: 2) }
+    let!(:pending_plate) { create(:plate_with_empty_wells, well_count: 2) }
+
+    before do
+      parent_of_passed_plate.wells.zip(passed_plate.wells).each do |source, target|
+        create(:transfer_request, asset: source, target_asset: target, state: 'passed')
+      end
+
+      parent_of_failed_plate.wells.zip(failed_plate.wells).each do |source, target|
+        create(:transfer_request, asset: source, target_asset: target, state: 'failed')
+      end
+
+      parent_of_pending_plate.wells.zip(pending_plate.wells).each do |source, target|
+        create(:transfer_request, asset: source, target_asset: target, state: 'pending')
+      end
+    end
+
+    context "when filtering by 'passed'" do
+      subject(:result) { Plate.in_state('passed') }
+
+      it 'returns plates whose transfer requests are in the passed state' do
+        expect(result).to include(passed_plate)
+      end
+
+      it 'does not return plates in other states' do
+        expect(result).not_to include(failed_plate, pending_plate)
+      end
+    end
+
+    context "when filtering by 'failed'" do
+      subject(:result) { Plate.in_state('failed') }
+
+      it 'returns plates whose transfer requests are in the failed state' do
+        expect(result).to include(failed_plate)
+      end
+
+      it 'does not return plates in other states' do
+        expect(result).not_to include(passed_plate, pending_plate)
+      end
+    end
+
+    context "when filtering by 'pending'" do
+      subject(:result) { Plate.in_state('pending') }
+
+      it 'returns plates whose transfer requests are in the pending state' do
+        expect(result).to include(pending_plate)
+      end
+
+      it 'does not return plates in other states' do
+        expect(result).not_to include(passed_plate, failed_plate)
+      end
+    end
+
+    context 'when filtering by multiple states' do
+      subject(:result) { Plate.in_state(%w[passed failed]) }
+
+      it 'returns plates matching any of the specified states' do
+        expect(result).to include(passed_plate, failed_plate)
+      end
+
+      it 'does not return plates not matching the specified states' do
+        expect(result).not_to include(pending_plate)
+      end
+    end
+
+    context 'when all valid states are passed' do
+      it 'returns all plates without filtering by transfer request state' do
+        expect(Plate.in_state(Transfer::State::ALL_STATES)).to include(passed_plate, failed_plate, pending_plate)
+      end
+    end
+
+    context 'when a plate has multiple transfer requests (one per well)' do
+      it 'does not return duplicate plates in the results' do
+        # passed_plate has 2 wells, so 2 transfer requests target it — one per well.
+        # The join in in_state produces one row per matching transfer request, so
+        # without a DISTINCT the plate appears twice in the results.
+        result = Plate.in_state('passed')
+        expect(result.count { |p| p == passed_plate }).to eq(1)
+      end
+    end
+  end
+end