diff --git a/core/src/main/java/org/elasticsearch/common/util/concurrent/ThreadContext.java b/core/src/main/java/org/elasticsearch/common/util/concurrent/ThreadContext.java
index 1ce119636f734..95c08e8889857 100644
--- a/core/src/main/java/org/elasticsearch/common/util/concurrent/ThreadContext.java
+++ b/core/src/main/java/org/elasticsearch/common/util/concurrent/ThreadContext.java
@@ -407,11 +407,10 @@ private ThreadContextStruct putHeaders(Map<String, String> headers) {
             if (headers.isEmpty()) {
                 return this;
             } else {
-                final Map<String, String> newHeaders = new HashMap<>();
+                final Map<String, String> newHeaders = new HashMap<>(this.requestHeaders);
                 for (Map.Entry<String, String> entry : headers.entrySet()) {
                     putSingleHeader(entry.getKey(), entry.getValue(), newHeaders);
                 }
-                newHeaders.putAll(this.requestHeaders);
                 return new ThreadContextStruct(newHeaders, responseHeaders, transientHeaders, isSystemContext);
             }
         }
diff --git a/core/src/test/java/org/elasticsearch/common/util/concurrent/ThreadContextTests.java b/core/src/test/java/org/elasticsearch/common/util/concurrent/ThreadContextTests.java
index bee56c229c02a..e71efa46424b2 100644
--- a/core/src/test/java/org/elasticsearch/common/util/concurrent/ThreadContextTests.java
+++ b/core/src/test/java/org/elasticsearch/common/util/concurrent/ThreadContextTests.java
@@ -29,7 +29,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.function.Supplier;
-
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasItem;
 import static org.hamcrest.Matchers.hasSize;
@@ -215,8 +214,8 @@ public void testResponseHeaders() {
     public void testCopyHeaders() {
         Settings build = Settings.builder().put("request.headers.default", "1").build();
         ThreadContext threadContext = new ThreadContext(build);
-        threadContext.copyHeaders(Collections.<String,String>emptyMap().entrySet());
-        threadContext.copyHeaders(Collections.<String,String>singletonMap("foo", "bar").entrySet());
+        threadContext.copyHeaders(Collections.<String, String>emptyMap().entrySet());
+        threadContext.copyHeaders(Collections.<String, String>singletonMap("foo", "bar").entrySet());
         assertEquals("bar", threadContext.getHeader("foo"));
     }
 
@@ -443,7 +442,7 @@ public void onAfter() {
                         assertEquals("bar", threadContext.getHeader("foo"));
                         assertEquals("bar_transient", threadContext.getTransient("foo"));
                         assertNotNull(threadContext.getTransient("failure"));
-                        assertEquals("exception from doRun", ((RuntimeException)threadContext.getTransient("failure")).getMessage());
+                        assertEquals("exception from doRun", ((RuntimeException) threadContext.getTransient("failure")).getMessage());
                         assertFalse(threadContext.isDefaultContext());
                         threadContext.putTransient("after", "after");
                     }
@@ -604,7 +603,7 @@ protected void doRun() throws Exception {
     public void testMarkAsSystemContext() throws IOException {
         try (ThreadContext threadContext = new ThreadContext(Settings.EMPTY)) {
             assertFalse(threadContext.isSystemContext());
-            try(ThreadContext.StoredContext context = threadContext.stashContext()){
+            try (ThreadContext.StoredContext context = threadContext.stashContext()) {
                 assertFalse(threadContext.isSystemContext());
                 threadContext.markAsSystemContext();
                 assertTrue(threadContext.isSystemContext());
@@ -613,6 +612,17 @@ public void testMarkAsSystemContext() throws IOException {
         }
     }
 
+    public void testPutHeaders() {
+        Settings build = Settings.builder().put("request.headers.default", "1").build();
+        ThreadContext threadContext = new ThreadContext(build);
+        threadContext.putHeader(Collections.<String, String>emptyMap());
+        threadContext.putHeader(Collections.<String, String>singletonMap("foo", "bar"));
+        assertEquals("bar", threadContext.getHeader("foo"));
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () ->
+            threadContext.putHeader(Collections.<String, String>singletonMap("foo", "boom")));
+        assertEquals("value for key [foo] already present", e.getMessage());
+    }
+
     /**
      * Sometimes wraps a Runnable in an AbstractRunnable.
      */