diff --git a/src/RestClient/Authenticators/OAuth/src/OAuthAuthenticator.cs b/src/RestClient/Authenticators/OAuth/src/OAuthAuthenticator.cs
index 68b5e88..44ecfcd 100644
--- a/src/RestClient/Authenticators/OAuth/src/OAuthAuthenticator.cs
+++ b/src/RestClient/Authenticators/OAuth/src/OAuthAuthenticator.cs
@@ -1,4 +1,4 @@
-namespace ClickView.Extensions.RestClient.Authenticators.OAuth;
+namespace ClickView.Extensions.RestClient.Authenticators.OAuth;
 
 using System;
 using System.Collections.Generic;
@@ -56,7 +56,7 @@ public virtual async Task AuthenticateAsync(IClientRequest request, Cancellation
             return;
         }
 
-        request.AddHeader("Authorization", "Bearer " + oAuthToken);
+        request.AddOrUpdateHeader("Authorization", "Bearer " + oAuthToken);
     }
 
     protected void AddTokenSource(ITokenSource tokenSource)
@@ -116,4 +116,4 @@ protected IAuthenticatorEndpointFactory CreateEndpointFactory(string endpoint)
 
         return new DefaultEndpointFactory(Options.Endpoints);
     }
-}
\ No newline at end of file
+}
diff --git a/src/RestClient/RestClient/src/Authentication/BasicAccessAuthenticator.cs b/src/RestClient/RestClient/src/Authentication/BasicAccessAuthenticator.cs
index b9664c4..e69409c 100644
--- a/src/RestClient/RestClient/src/Authentication/BasicAccessAuthenticator.cs
+++ b/src/RestClient/RestClient/src/Authentication/BasicAccessAuthenticator.cs
@@ -1,4 +1,4 @@
-namespace ClickView.Extensions.RestClient.Authentication;
+namespace ClickView.Extensions.RestClient.Authentication;
 
 using System;
 using System.Text;
@@ -24,7 +24,7 @@ public BasicAccessAuthenticator(string userId, string password)
 
     public Task AuthenticateAsync(IClientRequest request, CancellationToken token = default)
     {
-        request.AddHeader("Authorization", $"Basic {_encoded}");
+        request.AddOrUpdateHeader("Authorization", $"Basic {_encoded}");
         return Task.CompletedTask;
     }
 
@@ -33,4 +33,4 @@ private static string ToBase64(string s)
         var bytes = Encoding.UTF8.GetBytes(s);
         return Convert.ToBase64String(bytes);
     }
-}
\ No newline at end of file
+}
diff --git a/src/RestClient/RestClient/src/Authentication/HeaderAuthenticator.cs b/src/RestClient/RestClient/src/Authentication/HeaderAuthenticator.cs
index 0fb99ab..9eeeaca 100644
--- a/src/RestClient/RestClient/src/Authentication/HeaderAuthenticator.cs
+++ b/src/RestClient/RestClient/src/Authentication/HeaderAuthenticator.cs
@@ -1,4 +1,4 @@
-namespace ClickView.Extensions.RestClient.Authentication;
+namespace ClickView.Extensions.RestClient.Authentication;
 
 using System.Threading;
 using System.Threading.Tasks;
@@ -20,8 +20,8 @@ public HeaderAuthenticator(string key, string value)
 
     public Task AuthenticateAsync(IClientRequest request, CancellationToken token = default)
     {
-        request.AddHeader(_key, _value);
+        request.AddOrUpdateHeader(_key, _value);
 
         return Task.CompletedTask;
     }
-}
\ No newline at end of file
+}
diff --git a/src/RestClient/RestClient/src/Requests/BaseRestClientRequest.cs b/src/RestClient/RestClient/src/Requests/BaseRestClientRequest.cs
index 835a10a..535667f 100644
--- a/src/RestClient/RestClient/src/Requests/BaseRestClientRequest.cs
+++ b/src/RestClient/RestClient/src/Requests/BaseRestClientRequest.cs
@@ -45,6 +45,18 @@ public void AddHeader(string key, IEnumerable<string> values)
         _headers.Add(key, values);
     }
 
+    public void AddOrUpdateHeader(string key, string value)
+    {
+        _headers.Remove(key);   // no-op if it does not exist
+        _headers.Add(key, value);
+    }
+
+    public void AddOrUpdateHeader(string key, IEnumerable<string> values)
+    {
+        _headers.Remove(key);   // no-op if it does not exist
+        _headers.Add(key, values);
+    }
+
     public void AddBody(object body)
     {
         if (!MethodSupportsBody(Method))
@@ -53,7 +65,7 @@ public void AddBody(object body)
         _content = body;
         return;
 
-        bool MethodSupportsBody(HttpMethod method)
+        static bool MethodSupportsBody(HttpMethod method)
         {
             return method == HttpMethod.Post ||
 #if NETCOREAPP2_1_OR_GREATER
diff --git a/src/RestClient/RestClient/src/Requests/IClientRequest.cs b/src/RestClient/RestClient/src/Requests/IClientRequest.cs
index 23e262e..416ffc2 100644
--- a/src/RestClient/RestClient/src/Requests/IClientRequest.cs
+++ b/src/RestClient/RestClient/src/Requests/IClientRequest.cs
@@ -11,5 +11,7 @@ public interface IClientRequest
 
     void AddHeader(string key, string value);
     void AddHeader(string key, IEnumerable<string> values);
+    void AddOrUpdateHeader(string key, string value);
+    void AddOrUpdateHeader(string key, IEnumerable<string> values);
     void AddBody(object body);
-}
\ No newline at end of file
+}
diff --git a/src/RestClient/RestClient/src/RestClient.cs b/src/RestClient/RestClient/src/RestClient.cs
index 69c05c8..7a1539f 100644
--- a/src/RestClient/RestClient/src/RestClient.cs
+++ b/src/RestClient/RestClient/src/RestClient.cs
@@ -162,7 +162,6 @@ private static HttpClient CreateClient(RestClientOptions options)
 
         PrepareHttpClient(httpClient, options);
 
-
         return httpClient;
     }
 
diff --git a/src/RestClient/RestClient/test/RestClientRequestTests.cs b/src/RestClient/RestClient/test/RestClientRequestTests.cs
index 5cc7474..054d6b3 100644
--- a/src/RestClient/RestClient/test/RestClientRequestTests.cs
+++ b/src/RestClient/RestClient/test/RestClientRequestTests.cs
@@ -73,6 +73,32 @@ public void AddBody_GetRequest_ThrowsException()
         Assert.Equal("Cannot add body to GET", ex.Message);
     }
 
+    [Fact]
+    public void AddOrUpdateHeader_SingleValue_SameHeaderTwice_DoesNotThrow()
+    {
+        const string key = "X-Test-Header";
+        const string value = "TestValue";
+
+        var request = new RestClientRequest(HttpMethod.Post, "test");
+        request.AddOrUpdateHeader(key, value);
+        request.AddOrUpdateHeader(key, value);
+
+        Assert.Single(request.Headers);
+    }
+
+    [Fact]
+    public void AddOrUpdateHeader_MultipleValues_SameHeaderTwice_DoesNotThrow()
+    {
+        const string key = "X-Test-Header";
+        string[] values = ["TestValue1", "TestValue2"];
+
+        var request = new RestClientRequest(HttpMethod.Post, "test");
+        request.AddOrUpdateHeader(key, values);
+        request.AddOrUpdateHeader(key, values);
+
+        Assert.Single(request.Headers);
+    }
+
     private class TestSerializer : ISerializer
     {
         public string Format => "test";
diff --git a/src/RestClient/RestClient/test/RestClientTests.cs b/src/RestClient/RestClient/test/RestClientTests.cs
index 5cc8acc..0e08139 100644
--- a/src/RestClient/RestClient/test/RestClientTests.cs
+++ b/src/RestClient/RestClient/test/RestClientTests.cs
@@ -214,7 +214,7 @@ public async Task ExecuteAsync_DefaultUserAgentSet_RequestContainsDefaultUserAge
                 ItExpr.IsAny<HttpRequestMessage>(),
                 ItExpr.IsAny<CancellationToken>()
             )
-            .Callback<HttpRequestMessage, CancellationToken>((request, token) =>
+            .Callback<HttpRequestMessage, CancellationToken>((request, _) =>
             {
                 userAgentHeader = request.Headers.UserAgent.ToString();
             })
@@ -244,4 +244,4 @@ protected override async ValueTask<RestClientResponse<string>> ParseResponseAsyn
             return new RestClientResponse<string>(message, await message.Content.ReadAsStringAsync());
         }
     }
-}
\ No newline at end of file
+}