Controller (app/controllers/admin/deduplicate_patients_controller.rb):

app/controllers/admin/deduplicate_patients_controller.rb

@@ -1,18 +1,31 @@
 class Admin::DeduplicatePatientsController < AdminController
   skip_before_action :verify_authenticity_token
+  before_action :set_filter_options, only: [:show], if: :filter_enabled?
   DUPLICATE_LIMIT = 250
 
   def show
     facilities = current_admin.accessible_facilities(:manage)
     authorize { facilities.any? }
 
+    # Apply facility filter if selected and feature flag is enabled
+    filtered_facilities = if filter_enabled? && @selected_facility.present?
+      facilities.where(id: @selected_facility.id)
+    elsif filter_enabled? && @selected_district.present?
+      facilities.where(id: @selected_district.facilities)
+    else
+      facilities
+    end
+
     # Scoping by facilities is costly for users who have a lot of facilities
-    duplicate_patient_ids = if current_admin.accessible_organizations(:manage).any?
+    use_all_organizations = current_admin.accessible_organizations(:manage).any? &&
+      (!filter_enabled? || (!@selected_district.present? && !@selected_facility.present?))
+
+    duplicate_patient_ids = if use_all_organizations
       PatientDeduplication::Strategies.identifier_excluding_full_name_match(limit: DUPLICATE_LIMIT)
     else
       PatientDeduplication::Strategies.identifier_excluding_full_name_match_for_facilities(
         limit: DUPLICATE_LIMIT,
-        facilities: facilities
+        facilities: filtered_facilities
       )
     end
 
@@ -44,4 +57,42 @@ def can_admin_deduplicate_patients?(patients)
       .where(id: patients.pluck(:assigned_facility_id))
       .any?
   end
+
+  private
+
+  def set_filter_options
+    @accessible_facilities = current_admin.accessible_facilities(:manage)
+    populate_districts
+    set_selected_district
+    populate_facilities
+    set_selected_facility
+  end
+
+  def populate_districts
+    @districts = Region.district_regions
+      .joins("INNER JOIN regions facility_region ON regions.path @> facility_region.path")
+      .where("facility_region.source_id" => @accessible_facilities.map(&:id))
+      .distinct(:slug)
+      .order(:name)
+  end
+
+  def set_selected_district
+    @selected_district = if params[:district_slug].present?
+      @districts.find_by(slug: params[:district_slug])
+    elsif @districts.present?
+      @districts.first
+    end
+  end
+
+  def populate_facilities
+    @facilities = @accessible_facilities.where(id: @selected_district&.facilities).order(:name)
+  end
+
+  def set_selected_facility
+    @selected_facility = @facilities.find_by(id: params[:facility_id]) if params[:facility_id].present?
+  end
+
+  def filter_enabled?
+    Flipper.enabled?(:patient_deduplication_filter, current_admin)
+  end
 end

app/views/admin/deduplicate_patients/show.html.erb

@@ -1,4 +1,38 @@
 <h1 class="page-title mb-3">Merge duplicate patients</h1>
+
+<% if Flipper.enabled?(:patient_deduplication_filter, current_admin) %>
+  <%= bootstrap_form_with(url: admin_deduplication_path, method: :get, layout: :horizontal, class: "mb-4") do |form| %>
+    <% html_select_options = { onchange: "this.form.submit();" }
+       searchable_select_options = html_select_options.merge(class: "selectpicker", data: {live_search: true}) %>
+    <div class="form-row">
+      <div id="district-selector" class="form-group col-md-4">
+        <%= form.select :district_slug,
+                        @districts.order(:name).map { |district| [district.name, district.slug] },
+                        {
+                          hide_label: true,
+                          selected: @selected_district&.slug,
+                          wrapper: false
+                        },
+                        searchable_select_options
+        %>
+      </div>
+      <div id="facility-selector" class="form-group col-md-4">
+        <%= form.select :facility_id,
+                        @facilities.order(:name).map { |facility| [facility.label_with_district, facility.id] },
+                        {
+                            hide_label: true,
+                            include_blank: "All facilities",
+                            selected: @selected_facility&.id,
+                            wrapper: false
+                        },
+                        searchable_select_options
+        %>
+      </div>
+    </div>
+    <%= form.submit "Filter", style: "display: none" %>
+  <% end %>
+<% end %>
+
 <div class="card">
   <h3>
     <% if @duplicate_count == Admin::DeduplicatePatientsController::DUPLICATE_LIMIT %>