diff --git a/lib/optimizely/odp/odp_event.rb b/lib/optimizely/odp/odp_event.rb
index eff559b6..24e78ab9 100644
--- a/lib/optimizely/odp/odp_event.rb
+++ b/lib/optimizely/odp/odp_event.rb
@@ -39,7 +39,11 @@ def add_common_event_data(custom_data)
         data_source: 'ruby-sdk',
         data_source_version: VERSION
       }
-      data.update(custom_data)
+      # custom_data may use string keys (e.g. parsed from JSON); normalize those to symbols so
+      # they override the matching default key above instead of adding a duplicate-looking one,
+      # which later fails JSON generation (symbol/string key collision). Non-string keys are left
+      # as-is, since they can't collide with the symbol-keyed defaults and to_sym would raise.
+      data.update(custom_data.transform_keys { |key| key.is_a?(String) ? key.to_sym : key })
       data
     end
 
diff --git a/spec/odp/odp_event_spec.rb b/spec/odp/odp_event_spec.rb
new file mode 100644
index 00000000..87c13b83
--- /dev/null
+++ b/spec/odp/odp_event_spec.rb
@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+# Copyright 2026, Optimizely
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http:#www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+require 'spec_helper'
+require 'optimizely/odp/odp_event'
+
+describe Optimizely::OdpEvent do
+  describe 'custom data with string keys matching the default symbol-keyed fields' do
+    # String keys are what callers get back from parsed JSON (e.g. a testapp or any
+    # non-Ruby-native caller), which previously collided with the default symbol keys
+    # instead of overriding them, producing a hash with duplicate-looking keys that
+    # fails JSON generation (FSSDK-13349).
+    let(:custom_data) do
+      {
+        'data_source_type' => 'sdk',
+        'data_source' => 'test_sdk',
+        'data_source_version' => '0.0.1'
+      }
+    end
+
+    let(:event) { described_class.new(type: 'fullstack', action: 'test_action', identifiers: {'fs_user_id' => 'user1'}, data: custom_data) }
+
+    it 'overrides the default fields instead of duplicating them' do
+      parsed = JSON.parse(event.to_json)
+      expect(parsed['data']['data_source_type']).to eq('sdk')
+      expect(parsed['data']['data_source']).to eq('test_sdk')
+      expect(parsed['data']['data_source_version']).to eq('0.0.1')
+    end
+
+    it 'produces a hash with no duplicate symbol/string keys' do
+      data = event.instance_variable_get(:@data)
+      string_and_symbol_keys = data.keys.group_by(&:to_s)
+      expect(string_and_symbol_keys.values.map(&:size).uniq).to eq([1])
+    end
+
+    it 'serializes without raising' do
+      expect { event.to_json }.not_to raise_error
+    end
+  end
+
+  describe 'custom data with keys not present in the defaults' do
+    let(:event) { described_class.new(type: 'fullstack', action: 'test_action', identifiers: {'fs_user_id' => 'user1'}, data: {'custom_key' => 'custom_value'}) }
+
+    it 'keeps the defaults and adds the custom field' do
+      parsed = JSON.parse(event.to_json)
+      expect(parsed['data']['data_source_type']).to eq('sdk')
+      expect(parsed['data']['data_source']).to eq('ruby-sdk')
+      expect(parsed['data']['custom_key']).to eq('custom_value')
+    end
+  end
+
+  describe 'custom data with non-string keys' do
+    # odp_data_types_valid? only checks hash values, not keys, so a caller can reach
+    # OdpEvent with non-string keys (e.g. Integer, nil). These can't collide with the
+    # symbol-keyed defaults, and calling to_sym on them would raise, so they must be
+    # left as-is rather than symbolized.
+    let(:event) { described_class.new(type: 'fullstack', action: 'test_action', identifiers: {'fs_user_id' => 'user1'}, data: {1 => 'x', nil => 'y'}) }
+
+    it 'does not raise' do
+      expect { event.to_json }.not_to raise_error
+    end
+
+    it 'keeps the defaults and the non-string-keyed fields' do
+      parsed = JSON.parse(event.to_json)
+      expect(parsed['data']['data_source_type']).to eq('sdk')
+      expect(parsed['data']['data_source']).to eq('ruby-sdk')
+      expect(parsed['data']['1']).to eq('x')
+      expect(parsed['data']['']).to eq('y')
+    end
+  end
+end