diff --git a/http-cache-reqwest/Cargo.toml b/http-cache-reqwest/Cargo.toml
index 882de73..0a59db4 100644
--- a/http-cache-reqwest/Cargo.toml
+++ b/http-cache-reqwest/Cargo.toml
@@ -31,6 +31,7 @@ url = { version = "2.5.4", features = ["serde"] }
 
 # Optional dependencies for streaming feature
 futures-util = { version = "0.3.31", optional = true }
+httpdate = "1.0.3"
 
 [dependencies.http-cache]
 path = "../http-cache"
diff --git a/http-cache-reqwest/examples/reqwest_basic.rs b/http-cache-reqwest/examples/reqwest_basic.rs
index 1bc25d3..bd20730 100644
--- a/http-cache-reqwest/examples/reqwest_basic.rs
+++ b/http-cache-reqwest/examples/reqwest_basic.rs
@@ -8,6 +8,7 @@ use reqwest::Client;
 use reqwest_middleware::ClientBuilder;
 use std::time::Instant;
 use wiremock::{matchers::method, Mock, MockServer, ResponseTemplate};
+use wiremock::matchers::header;
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
diff --git a/http-cache-reqwest/src/lib.rs b/http-cache-reqwest/src/lib.rs
index 681bfd2..b3bb3ae 100644
--- a/http-cache-reqwest/src/lib.rs
+++ b/http-cache-reqwest/src/lib.rs
@@ -403,7 +403,7 @@ impl Middleware for ReqwestMiddleware<'_> {
     }
     fn update_headers(&mut self, parts: &Parts) -> Result<()> {
         for header in parts.headers.iter() {
-            self.req.headers_mut().append(header.0.clone(), header.1.clone());
+            self.req.headers_mut().insert(header.0.clone(), header.1.clone());
         }
         Ok(())
     }
diff --git a/http-cache-reqwest/src/test.rs b/http-cache-reqwest/src/test.rs
index abc23db..59fce5b 100644
--- a/http-cache-reqwest/src/test.rs
+++ b/http-cache-reqwest/src/test.rs
@@ -1,12 +1,14 @@
 use crate::{BadRequest, Cache, HttpCacheError};
 use std::sync::Arc;
-
+use std::time::{Duration, SystemTime};
 use http_cache::*;
 use reqwest::Client;
 use reqwest_middleware::ClientBuilder;
+use tokio::time::sleep;
 #[cfg(any(feature = "streaming", feature = "rate-limiting"))]
 use wiremock::matchers::path;
 use wiremock::{matchers::method, Mock, MockServer, ResponseTemplate};
+use wiremock::matchers::header;
 
 /// Helper function to create a temporary cache manager
 fn create_cache_manager() -> CACacheManager {
@@ -704,6 +706,49 @@ async fn revalidation_200() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn duplicate_header() -> Result<()> {
+    let now = SystemTime::now();
+    let expires = SystemTime::now() + Duration::from_secs(2);
+
+    let mock_server = MockServer::start().await;
+    let mock = Mock::given(method(GET)).and(header("x-test", "test"))
+        .respond_with(
+            ResponseTemplate::new(200)
+                .insert_header("expires", httpdate::fmt_http_date(now))
+                .set_body_bytes("ok"),
+        )
+        .expect(2);
+    let mock_guard = mock_server.register_as_scoped(mock).await;
+    let url = format!("{}/", &mock_server.uri());
+    let manager = create_cache_manager();
+
+    // Construct reqwest client with cache defaults
+    let client = ClientBuilder::new(Client::new())
+        .with(Cache(HttpCache {
+            mode: CacheMode::Default,
+            manager: manager.clone(),
+            options: Default::default(),
+        }))
+        .build();
+
+    let resp1 = client.get(url.clone()).header("x-test", "test").send().await?;
+
+    let wait_until = expires + Duration::from_secs(2);
+    while SystemTime::now() < wait_until {
+        sleep(Duration::from_millis(100)).await;
+    }
+
+    let resp2 = client.get(url.clone()).header("x-test", "test").send().await?;
+
+    drop(mock_guard);
+
+    assert_eq!(resp1.text().await?, "ok");
+    assert_eq!(resp2.text().await?, "ok");
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn revalidation_500() -> Result<()> {
     let mock_server = MockServer::start().await;