diff --git a/config/feature_flags/development/ci_includes_count_duplicates.yml b/config/feature_flags/development/ci_includes_count_duplicates.yml
new file mode 100644
index 0000000000000000000000000000000000000000..5e33edddc03a9f79c0e2cbbbb4a3155d29d43008
--- /dev/null
+++ b/config/feature_flags/development/ci_includes_count_duplicates.yml
@@ -0,0 +1,8 @@
+---
+name: ci_includes_count_duplicates
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/111726
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/391517
+milestone: '15.9'
+type: development
+group: group::pipeline authoring
+default_enabled: false
diff --git a/lib/gitlab/ci/config/external/context.rb b/lib/gitlab/ci/config/external/context.rb
index 138e79db33181d05a03d015ac46dc346070be3f4..6eef279d3de843396efe9d3e6399babb1bea6f5f 100644
--- a/lib/gitlab/ci/config/external/context.rb
+++ b/lib/gitlab/ci/config/external/context.rb
@@ -10,6 +10,7 @@ class Context
           TimeoutError = Class.new(StandardError)
 
           MAX_INCLUDES = 100
+          NEW_MAX_INCLUDES = 150 # Update to MAX_INCLUDES when FF ci_includes_count_duplicates is removed
 
           include ::Gitlab::Utils::StrongMemoize
 
@@ -27,10 +28,10 @@ def initialize(
             @user = user
             @parent_pipeline = parent_pipeline
             @variables = variables || Ci::Variables::Collection.new
-            @expandset = Set.new
+            @expandset = Feature.enabled?(:ci_includes_count_duplicates, project) ? [] : Set.new
             @execution_deadline = 0
             @logger = logger || Gitlab::Ci::Pipeline::Logger.new(project: project)
-            @max_includes = MAX_INCLUDES
+            @max_includes = Feature.enabled?(:ci_includes_count_duplicates, project) ? NEW_MAX_INCLUDES : MAX_INCLUDES
             yield self if block_given?
           end
 
diff --git a/lib/gitlab/ci/config/external/mapper/verifier.rb b/lib/gitlab/ci/config/external/mapper/verifier.rb
index 6c161e9515427f8f49a3dc163133e0475040ed61..83ef58bb46f8c9dd3f8d4e3df9c3816233c4a6ff 100644
--- a/lib/gitlab/ci/config/external/mapper/verifier.rb
+++ b/lib/gitlab/ci/config/external/mapper/verifier.rb
@@ -28,7 +28,11 @@ def process_without_instrumentation(files)
                 file.validate_content! if file.valid?
                 file.load_and_validate_expanded_hash! if file.valid?
 
-                context.expandset.add(file)
+                if ::Feature.enabled?(:ci_includes_count_duplicates, context.project)
+                  context.expandset << file
+                else
+                  context.expandset.add(file)
+                end
               end
             end
 
diff --git a/spec/lib/gitlab/ci/config/external/context_spec.rb b/spec/lib/gitlab/ci/config/external/context_spec.rb
index 45efb16434b8c8616c85fef0d329e92c7144b4b9..1fd3cf3c99fc58e6ba4c90795d414c09ca20a08f 100644
--- a/spec/lib/gitlab/ci/config/external/context_spec.rb
+++ b/spec/lib/gitlab/ci/config/external/context_spec.rb
@@ -14,7 +14,8 @@
   describe 'attributes' do
     context 'with values' do
       it { is_expected.to have_attributes(**attributes) }
-      it { expect(subject.expandset).to eq(Set.new) }
+      it { expect(subject.expandset).to eq([]) }
+      it { expect(subject.max_includes).to eq(Gitlab::Ci::Config::External::Context::NEW_MAX_INCLUDES) }
       it { expect(subject.execution_deadline).to eq(0) }
       it { expect(subject.variables).to be_instance_of(Gitlab::Ci::Variables::Collection) }
       it { expect(subject.variables_hash).to be_instance_of(ActiveSupport::HashWithIndifferentAccess) }
@@ -25,11 +26,39 @@
       let(:attributes) { { project: nil, user: nil, sha: nil } }
 
       it { is_expected.to have_attributes(**attributes) }
-      it { expect(subject.expandset).to eq(Set.new) }
+      it { expect(subject.expandset).to eq([]) }
+      it { expect(subject.max_includes).to eq(Gitlab::Ci::Config::External::Context::NEW_MAX_INCLUDES) }
       it { expect(subject.execution_deadline).to eq(0) }
       it { expect(subject.variables).to be_instance_of(Gitlab::Ci::Variables::Collection) }
       it { expect(subject.variables_hash).to be_instance_of(ActiveSupport::HashWithIndifferentAccess) }
     end
+
+    context 'when FF ci_includes_count_duplicates is disabled' do
+      before do
+        stub_feature_flags(ci_includes_count_duplicates: false)
+      end
+
+      context 'with values' do
+        it { is_expected.to have_attributes(**attributes) }
+        it { expect(subject.expandset).to eq(Set.new) }
+        it { expect(subject.max_includes).to eq(Gitlab::Ci::Config::External::Context::MAX_INCLUDES) }
+        it { expect(subject.execution_deadline).to eq(0) }
+        it { expect(subject.variables).to be_instance_of(Gitlab::Ci::Variables::Collection) }
+        it { expect(subject.variables_hash).to be_instance_of(ActiveSupport::HashWithIndifferentAccess) }
+        it { expect(subject.variables_hash).to include('a' => 'b') }
+      end
+
+      context 'without values' do
+        let(:attributes) { { project: nil, user: nil, sha: nil } }
+
+        it { is_expected.to have_attributes(**attributes) }
+        it { expect(subject.expandset).to eq(Set.new) }
+        it { expect(subject.max_includes).to eq(Gitlab::Ci::Config::External::Context::MAX_INCLUDES) }
+        it { expect(subject.execution_deadline).to eq(0) }
+        it { expect(subject.variables).to be_instance_of(Gitlab::Ci::Variables::Collection) }
+        it { expect(subject.variables_hash).to be_instance_of(ActiveSupport::HashWithIndifferentAccess) }
+      end
+    end
   end
 
   describe '#set_deadline' do
diff --git a/spec/lib/gitlab/ci/config/external/mapper/verifier_spec.rb b/spec/lib/gitlab/ci/config/external/mapper/verifier_spec.rb
index 7bdd519cb74de27ac57fc4eb4b9cd229a3579153..a219666f24e7d4484f3c0ca1dcb9a331547e58b0 100644
--- a/spec/lib/gitlab/ci/config/external/mapper/verifier_spec.rb
+++ b/spec/lib/gitlab/ci/config/external/mapper/verifier_spec.rb
@@ -150,7 +150,7 @@
       end
     end
 
-    context 'when max_includes is exceeded' do
+    context 'when total file count exceeds max_includes' do
       context 'when files are nested' do
         let(:files) do
           [
@@ -158,11 +158,8 @@
           ]
         end
 
-        before do
-          allow(context).to receive(:max_includes).and_return(1)
-        end
-
         it 'raises Processor::IncludeError' do
+          allow(context).to receive(:max_includes).and_return(1)
           expect { process }.to raise_error(Gitlab::Ci::Config::External::Processor::IncludeError)
         end
       end
@@ -175,13 +172,36 @@
           ]
         end
 
-        before do
+        it 'raises Mapper::TooManyIncludesError' do
           allow(context).to receive(:max_includes).and_return(1)
+          expect { process }.to raise_error(Gitlab::Ci::Config::External::Mapper::TooManyIncludesError)
         end
+      end
 
-        it 'raises Mapper::TooManyIncludesError' do
+      context 'when files are duplicates' do
+        let(:files) do
+          [
+            Gitlab::Ci::Config::External::File::Local.new({ local: 'myfolder/file1.yml' }, context),
+            Gitlab::Ci::Config::External::File::Local.new({ local: 'myfolder/file1.yml' }, context),
+            Gitlab::Ci::Config::External::File::Local.new({ local: 'myfolder/file1.yml' }, context)
+          ]
+        end
+
+        it 'raises error' do
+          allow(context).to receive(:max_includes).and_return(2)
           expect { process }.to raise_error(Gitlab::Ci::Config::External::Mapper::TooManyIncludesError)
         end
+
+        context 'when FF ci_includes_count_duplicates is disabled' do
+          before do
+            stub_feature_flags(ci_includes_count_duplicates: false)
+          end
+
+          it 'does not raise error' do
+            allow(context).to receive(:max_includes).and_return(2)
+            expect { process }.not_to raise_error
+          end
+        end
       end
     end
   end
diff --git a/spec/lib/gitlab/ci/config/external/mapper_spec.rb b/spec/lib/gitlab/ci/config/external/mapper_spec.rb
index dc14bf18b982b9385a80f832c3d55fe6e4292f54..b3115617084b728c3a08ec7b3a38c8159d0f9f5b 100644
--- a/spec/lib/gitlab/ci/config/external/mapper_spec.rb
+++ b/spec/lib/gitlab/ci/config/external/mapper_spec.rb
@@ -230,9 +230,20 @@
         expect { process }.not_to raise_error
       end
 
-      it 'has expanset with one' do
+      it 'has expanset with two' do
         process
-        expect(context.expandset.size).to eq(1)
+        expect(context.expandset.size).to eq(2)
+      end
+
+      context 'when FF ci_includes_count_duplicates is disabled' do
+        before do
+          stub_feature_flags(ci_includes_count_duplicates: false)
+        end
+
+        it 'has expanset with one' do
+          process
+          expect(context.expandset.size).to eq(1)
+        end
       end
     end