Fix RUBY-2030 Client/Cluster construction fails when duplicate seed addresses are provided (#1594)

* Ensure that a duplicate host entry won't prevent a Cluster from instantiating.

* Some pull request updates for tests.

* Disconnect monitoring threads.

* Use register_cluster.

Author: jonhyman

lib/mongo/cluster.rb

@@ -109,6 +109,8 @@ def initialize(seeds, monitoring, options = Options::Redacted.new)
         options[:cleanup] = false
       end
 
+      seeds = seeds.uniq
+
       @servers = []
       @monitoring = monitoring
       @event_listeners = Event::Listeners.new

spec/mongo/cluster_spec.rb

@@ -20,6 +20,25 @@
 
   let(:cluster) { cluster_without_io }
 
+  describe 'initialize' do
+
+    context 'when there are duplicate addresses' do
+
+      let(:addresses) do
+        SpecConfig.instance.addresses + SpecConfig.instance.addresses
+      end
+      let(:cluster_with_dup_addresses) do
+        register_cluster(
+          described_class.new(addresses, monitoring, SpecConfig.instance.test_options))
+      end
+
+      it 'does not raise an exception' do
+        expect { cluster_with_dup_addresses }.not_to raise_error
+      end
+    end
+
+  end
+
   describe '#==' do
 
     context 'when the other is a cluster' do