diff --git a/CHANGELOG.md b/CHANGELOG.md
index 88894b8..882bc70 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+1. fix: Warn when multiple PostHog client instances are created with the same API key ([#57](https://github.com/PostHog/posthog-ruby/issues/57))
+   - Multiple instances can cause dropped events and inconsistent behavior
+   - Use `disable_singleton_warning: true` when intentionally creating multiple clients (e.g., for different projects)
+   - Documentation updated with singleton best practices
+
 ## 3.5.5 - 2026-03-04
 
 1. feat: Add semver comparison operators for local feature flag evaluation ([#107](https://github.com/PostHog/posthog-ruby/pull/107))
diff --git a/README.md b/README.md
index 430ff63..18ba223 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,9 @@ Please see the main [PostHog docs](https://posthog.com/docs).
 
 Specifically, the [Ruby integration](https://posthog.com/docs/integrations/ruby-integration) details.
 
+> [!IMPORTANT]
+> **Use a single client instance (singleton)** — Create the PostHog client once and reuse it throughout your application. Multiple client instances with the same API key can cause dropped events and inconsistent behavior. The SDK will log a warning if it detects multiple instances. For Rails apps, use `PostHog.init` in an initializer (see [posthog-rails](posthog-rails/README.md)).
+
 > [!IMPORTANT]
 > Supports Ruby 3.2 and above
 >
diff --git a/lib/posthog/client.rb b/lib/posthog/client.rb
index 8e9355c..c9fc619 100644
--- a/lib/posthog/client.rb
+++ b/lib/posthog/client.rb
@@ -17,6 +17,35 @@ class Client
     include PostHog::Utils
     include PostHog::Logging
 
+    # Thread-safe tracking of client instances per API key for singleton warnings
+    @instances_by_api_key = {}
+    @instances_mutex = Mutex.new
+
+    class << self
+      # Resets instance tracking. Used primarily for testing.
+      # In production, instance counts persist for the lifetime of the process.
+      def reset_instance_tracking!
+        @instances_mutex.synchronize do
+          @instances_by_api_key = {}
+        end
+      end
+
+      def _increment_instance_count(api_key)
+        @instances_mutex.synchronize do
+          count = @instances_by_api_key[api_key] || 0
+          @instances_by_api_key[api_key] = count + 1
+          count
+        end
+      end
+
+      def _decrement_instance_count(api_key)
+        @instances_mutex.synchronize do
+          count = (@instances_by_api_key[api_key] || 1) - 1
+          @instances_by_api_key[api_key] = [count, 0].max
+        end
+      end
+    end
+
     # @param [Hash] opts
     # @option opts [String] :api_key Your project's api_key
     # @option opts [String] :personal_api_key Your personal API key
@@ -32,6 +61,8 @@ class Client
     #   Measured in seconds, defaults to 3.
     # @option opts [Proc] :before_send A block that receives the event hash and should return either a modified hash
     #   to be sent to PostHog or nil to prevent the event from being sent. e.g. `before_send: ->(event) { event }`
+    # @option opts [Bool] :disable_singleton_warning +true+ to suppress the warning when multiple clients
+    #   share the same API key. Use only when you intentionally need multiple clients. Defaults to +false+.
     def initialize(opts = {})
       symbolize_keys!(opts)
 
@@ -52,6 +83,19 @@ def initialize(opts = {})
 
       check_api_key!
 
+      # Warn when multiple clients are created with the same API key (can cause dropped events)
+      unless opts[:test_mode] || opts[:disable_singleton_warning]
+        previous_count = self.class._increment_instance_count(@api_key)
+        if previous_count >= 1
+          logger.warn(
+            'Multiple PostHog client instances detected for the same API key. ' \
+            'This can cause dropped events and inconsistent behavior. ' \
+            'Use a singleton pattern: instantiate once and reuse the client. ' \
+            'See https://posthog.com/docs/libraries/ruby'
+          )
+        end
+      end
+
       @feature_flags_poller =
         FeatureFlagsPoller.new(
           opts[:feature_flags_polling_interval],
@@ -444,6 +488,7 @@ def reload_feature_flags
     end
 
     def shutdown
+      self.class._decrement_instance_count(@api_key) if @api_key
       @feature_flags_poller.shutdown_poller
       flush
     end
diff --git a/posthog-rails/README.md b/posthog-rails/README.md
index 78c1292..2686984 100644
--- a/posthog-rails/README.md
+++ b/posthog-rails/README.md
@@ -40,6 +40,8 @@ This will create `config/initializers/posthog.rb` with sensible defaults and doc
 
 ## Configuration
 
+PostHog.init creates a single client instance used across your app. Avoid creating multiple `PostHog::Client` instances with the same API key — it can cause dropped events.
+
 The generated initializer at `config/initializers/posthog.rb` includes all available options:
 
 ```ruby
diff --git a/posthog-rails/lib/generators/posthog/templates/posthog.rb b/posthog-rails/lib/generators/posthog/templates/posthog.rb
index 3b6fc81..2f0ca2d 100644
--- a/posthog-rails/lib/generators/posthog/templates/posthog.rb
+++ b/posthog-rails/lib/generators/posthog/templates/posthog.rb
@@ -46,6 +46,8 @@
 # CORE POSTHOG CONFIGURATION
 # ============================================================================
 # Initialize the PostHog client with core SDK options.
+# IMPORTANT: Use PostHog.init once — creating multiple clients can cause dropped events.
+# PostHog.init provides a singleton-like pattern; use PostHog.capture, PostHog.identify, etc.
 
 PostHog.init do |config|
   # ============================================================================
diff --git a/spec/posthog/client_spec.rb b/spec/posthog/client_spec.rb
index c01b19a..002486f 100644
--- a/spec/posthog/client_spec.rb
+++ b/spec/posthog/client_spec.rb
@@ -36,6 +36,61 @@ module PostHog
                                                            skip_ssl_verification: true })
         expect { Client.new api_key: API_KEY, skip_ssl_verification: true }.to_not raise_error
       end
+
+      context 'singleton warning' do
+        before do
+          # Stub HTTP to allow creating clients without test_mode (which triggers the warning)
+          stub_request(:post, 'https://app.posthog.com/batch/').to_return(status: 200, body: '{}')
+          stub_request(:get, %r{https://app\.posthog\.com/api/feature_flag/}).to_return(status: 200, body: '{}')
+        end
+
+        it 'warns when multiple clients are created with the same API key' do
+          Client.new(api_key: API_KEY, test_mode: false)
+          Client.new(api_key: API_KEY, test_mode: false)
+
+          expect(logger).to have_received(:warn).with(
+            include('Multiple PostHog client instances detected')
+          )
+          expect(logger).to have_received(:warn).with(
+            include('singleton pattern')
+          )
+        end
+
+        it 'does not warn when only one client exists' do
+          Client.new(api_key: API_KEY, test_mode: false)
+
+          expect(logger).not_to have_received(:warn).with(
+            include('Multiple PostHog client instances detected')
+          )
+        end
+
+        it 'does not warn when disable_singleton_warning is true' do
+          Client.new(api_key: API_KEY, test_mode: false)
+          Client.new(api_key: API_KEY, test_mode: false, disable_singleton_warning: true)
+
+          expect(logger).not_to have_received(:warn).with(
+            include('Multiple PostHog client instances detected')
+          )
+        end
+
+        it 'does not warn when clients use different API keys' do
+          Client.new(api_key: API_KEY, test_mode: false)
+          Client.new(api_key: 'different_key', test_mode: false)
+
+          expect(logger).not_to have_received(:warn).with(
+            include('Multiple PostHog client instances detected')
+          )
+        end
+
+        it 'does not warn when test_mode is true' do
+          Client.new(api_key: API_KEY, test_mode: true)
+          Client.new(api_key: API_KEY, test_mode: true)
+
+          expect(logger).not_to have_received(:warn).with(
+            include('Multiple PostHog client instances detected')
+          )
+        end
+      end
     end
 
     describe '#capture' do
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index cdc3288..c227ba0 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -7,6 +7,7 @@
 RSpec.configure do |config|
   config.before(:each) do
     PostHog::Logging.logger = Logger.new(File::NULL) # Suppress all logging
+    PostHog::Client.reset_instance_tracking!
   end
 end