diff --git a/activerecord/lib/active_record/connection_adapters/pool_manager.rb b/activerecord/lib/active_record/connection_adapters/pool_manager.rb
index 06d8cbc135e81..cb09694327886 100644
--- a/activerecord/lib/active_record/connection_adapters/pool_manager.rb
+++ b/activerecord/lib/active_record/connection_adapters/pool_manager.rb
@@ -8,7 +8,7 @@ def initialize
       end
 
       def shard_names
-        @role_to_shard_mapping.values.flat_map { |shard_map| shard_map.keys }
+        @role_to_shard_mapping.values.flat_map { |shard_map| shard_map.keys }.uniq
       end
 
       def role_names
diff --git a/activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb b/activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb
index 6df45a8312af3..1e887c0d8b4fe 100644
--- a/activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb
+++ b/activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb
@@ -38,13 +38,14 @@ def test_establish_connection_using_3_levels_config
           @prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, config
 
           ActiveRecord::Base.connects_to(shards: {
-            default: { writing: :primary },
-            shard_one: { writing: :primary_shard_one }
+            default: { writing: :primary, reading: :primary },
+            shard_one: { writing: :primary_shard_one, reading: :primary_shard_one }
           })
 
           base_pool = ActiveRecord::Base.connection_handler.retrieve_connection_pool("ActiveRecord::Base")
           default_pool = ActiveRecord::Base.connection_handler.retrieve_connection_pool("ActiveRecord::Base", shard: :default)
 
+          assert_equal [:default, :shard_one], ActiveRecord::Base.connection_handler.send(:get_pool_manager, "ActiveRecord::Base").shard_names
           assert_equal base_pool, default_pool
           assert_equal "test/db/primary.sqlite3", default_pool.db_config.database
           assert_equal "primary", default_pool.db_config.name