diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7f8a3f27..8b613a7c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 ## [Unreleased]
 
+- Fix: `Shoryuken::Client.queues` no longer builds the same queue more than once under concurrency (mensfeld)
+  - The cache used an unsynchronized `@@queues[name] ||= Shoryuken::Queue.new(...)`. Building a queue makes
+    SQS API calls, and that I/O releases the GVL, so concurrent first-access (dispatch, processor-completion
+    and worker threads all call it) built the queue multiple times - redundant API calls, and a corrupt cache
+    on JRuby/TruffleRuby
+  - Access to the cache is now guarded by a mutex
+
 - Fix: Polling strategies are now thread-safe, and WeightedRoundRobin unpauses processed queues reliably (mensfeld)
   - `message_processed` runs on processor-completion threads (for FIFO queues) while `next_queue`/`messages_found`
     run on the dispatch thread; they mutate the same state with no synchronization, which is benign on MRI (GVL)
diff --git a/lib/shoryuken/client.rb b/lib/shoryuken/client.rb
index 0a4bc6a3..663fa62a 100644
--- a/lib/shoryuken/client.rb
+++ b/lib/shoryuken/client.rb
@@ -7,13 +7,19 @@ class Client
     # @return [Hash{String => Shoryuken::Queue}] cached queue instances by name
     @@queues = {}
 
+    # Guards the queue cache. queues is called concurrently from the dispatch
+    # thread, processor-completion threads and worker threads, and building a
+    # Shoryuken::Queue makes SQS API calls, so an unsynchronized `||=` would let
+    # several callers build the same queue (and corrupt the hash on JRuby).
+    @@queues_mutex = Mutex.new
+
     class << self
       # Returns a Queue instance for the given queue name
       #
       # @param name [String, Symbol] the name of the queue
       # @return [Shoryuken::Queue] the queue instance
       def queues(name)
-        @@queues[name.to_s] ||= Shoryuken::Queue.new(sqs, name)
+        @@queues_mutex.synchronize { @@queues[name.to_s] ||= Shoryuken::Queue.new(sqs, name) }
       end
 
       # Returns the current SQS client
@@ -30,7 +36,7 @@ def sqs
       def sqs=(sqs)
         # Since the @@queues values (Shoryuken::Queue objects) are built referencing @@sqs, if it changes, we need to
         #   re-build them on subsequent calls to `.queues(name)`.
-        @@queues = {}
+        @@queues_mutex.synchronize { @@queues = {} }
 
         Shoryuken.sqs_client = sqs
       end
diff --git a/spec/lib/shoryuken/client_spec.rb b/spec/lib/shoryuken/client_spec.rb
index 52b2de57..4909ec9e 100644
--- a/spec/lib/shoryuken/client_spec.rb
+++ b/spec/lib/shoryuken/client_spec.rb
@@ -17,5 +17,23 @@
       expect(Shoryuken::Client.queues(queue_name).url).to eq queue_url
       expect(Shoryuken::Client.queues(queue_name).url).to eq queue_url
     end
+
+    it 'constructs each queue only once under concurrent first access' do
+      allow(described_class).to receive(:sqs).and_return(sqs)
+
+      construction_count = Shoryuken::Helpers::AtomicCounter.new(0)
+      allow(Shoryuken::Queue).to receive(:new) do
+        construction_count.increment
+        # Mimic the SQS API latency during construction. sleep releases the GVL,
+        # so without synchronization every concurrent caller builds its own queue.
+        sleep 0.05
+        instance_double(Shoryuken::Queue)
+      end
+
+      threads = Array.new(10) { Thread.new { described_class.queues('concurrent') } }
+      threads.each(&:join)
+
+      expect(construction_count.value).to eq(1)
+    end
   end
 end