Only add product use case header if not present

Author: timgrein

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/action/BaseTransportInferenceAction.java

@@ -150,7 +150,13 @@ protected void doExecute(Task task, Request request, ActionListener<InferenceAct
             // We want to pass InferenceContext through the various infer methods in InferenceService in the long term
             var context = request.getContext();
             if (Objects.nonNull(context)) {
-                threadPool.getThreadContext().putHeader(InferencePlugin.X_ELASTIC_PRODUCT_USE_CASE_HTTP_HEADER, context.productUseCase());
+                var headerNotPresentInThreadContext = Objects.isNull(
+                    threadPool.getThreadContext().getHeader(InferencePlugin.X_ELASTIC_PRODUCT_USE_CASE_HTTP_HEADER)
+                );
+                if (headerNotPresentInThreadContext) {
+                    threadPool.getThreadContext()
+                        .putHeader(InferencePlugin.X_ELASTIC_PRODUCT_USE_CASE_HTTP_HEADER, context.productUseCase());
+                }
             }
 
             var service = serviceRegistry.getService(serviceName).get();