diff --git a/docs/usage.md b/docs/usage.md
index 424582f78..b84918e2a 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -594,7 +594,7 @@ One way of doing it is to use `RestClient` constructors that accept an instance
 
 - `BaseAddress` will be used to set the base address of the `HttpClient` instance if base address is not set there already.
 - `MaxTimeout`
-- `UserAgent` will be set if the `User-Agent` header is not set on the `HttpClient` instance already.
+- `UserAgent` will be added to the `RestClient.DefaultParameters` list as a HTTP header. This will be added to each request made by the `RestClient`, and the `HttpClient` instance will not be modified. This is to allow the `HttpClient` instance to be reused for scenarios where different `User-Agent` headers are required.
 - `Expect100Continue`
 
 Another option is to use a simple HTTP client factory as described [above](#simple-factory). 
diff --git a/src/RestSharp/RestClient.cs b/src/RestSharp/RestClient.cs
index d4ae91867..ccaae9f21 100644
--- a/src/RestSharp/RestClient.cs
+++ b/src/RestSharp/RestClient.cs
@@ -73,6 +73,7 @@ public RestClient(
 
         ConfigureSerializers(configureSerialization);
         Options = new ReadOnlyRestClientOptions(options);
+        DefaultParameters = new DefaultParameters(Options);
 
         if (useClientFactory) {
             _disposeHttpClient = false;
@@ -83,8 +84,6 @@ public RestClient(
             HttpClient         = GetClient();
         }
 
-        DefaultParameters = new DefaultParameters(Options);
-
         HttpClient GetClient() {
             var handler = new HttpClientHandler();
             ConfigureHttpMessageHandler(handler, Options);
@@ -92,6 +91,7 @@ HttpClient GetClient() {
 
             var httpClient = new HttpClient(finalHandler);
             ConfigureHttpClient(httpClient, options);
+            ConfigureDefaultParameters(options);
             configureDefaultHeaders?.Invoke(httpClient.DefaultRequestHeaders);
             return httpClient;
         }
@@ -181,7 +181,10 @@ public RestClient(
         Options           = new ReadOnlyRestClientOptions(opt);
         DefaultParameters = new DefaultParameters(Options);
 
-        if (options != null) ConfigureHttpClient(httpClient, options);
+        if (options != null) {
+            ConfigureHttpClient(httpClient, options);
+            ConfigureDefaultParameters(options);
+        }
     }
 
     /// <summary>
@@ -218,11 +221,6 @@ public RestClient(
     static void ConfigureHttpClient(HttpClient httpClient, RestClientOptions options) {
         if (options.MaxTimeout > 0) httpClient.Timeout = TimeSpan.FromMilliseconds(options.MaxTimeout);
 
-        if (options.UserAgent != null &&
-            httpClient.DefaultRequestHeaders.UserAgent.All(x => $"{x.Product?.Name}/{x.Product?.Version}" != options.UserAgent)) {
-            httpClient.DefaultRequestHeaders.TryAddWithoutValidation(KnownHeaders.UserAgent, options.UserAgent);
-        }
-
         if (options.Expect100Continue != null) httpClient.DefaultRequestHeaders.ExpectContinue = options.Expect100Continue;
     }
 
@@ -270,6 +268,15 @@ void ConfigureSerializers(ConfigureSerialization? configureSerialization) {
         AcceptedContentTypes = Serializers.GetAcceptedContentTypes();
     }
 
+    void ConfigureDefaultParameters(RestClientOptions options) {
+        if (options.UserAgent != null) {
+            if (!options.AllowMultipleDefaultParametersWithSameName
+                && DefaultParameters.Any(parameter => parameter.Type == ParameterType.HttpHeader && parameter.Name == KnownHeaders.UserAgent))
+                DefaultParameters.RemoveParameter(KnownHeaders.UserAgent, ParameterType.HttpHeader);
+            DefaultParameters.AddParameter(Parameter.CreateParameter(KnownHeaders.UserAgent, options.UserAgent, ParameterType.HttpHeader));
+        }
+    }
+
     readonly bool _disposeHttpClient;
 
     bool _disposed;
diff --git a/test/RestSharp.Tests/RestClientTests.cs b/test/RestSharp.Tests/RestClientTests.cs
index 276f1e92a..0bd5dd073 100644
--- a/test/RestSharp.Tests/RestClientTests.cs
+++ b/test/RestSharp.Tests/RestClientTests.cs
@@ -118,20 +118,41 @@ public void Should_use_new_httpClient_instance() {
     }
 
     [Fact]
-    public void ConfigureHttpClient_does_not_duplicate_user_agent_for_same_client() {
+    public void ConfigureDefaultParameters_sets_user_agent_new_httpClient_instance() {
+        // arrange
+        var clientOptions = new RestClientOptions();
+
+        // act
+        var restClient = new RestClient(clientOptions);
+
+        //assert
+        Assert.Single(
+            restClient.DefaultParameters,
+            parameter => parameter.Type == ParameterType.HttpHeader &&
+                parameter.Name == KnownHeaders.UserAgent &&
+                parameter.Value is string valueAsString &&
+                valueAsString == clientOptions.UserAgent);
+
+        Assert.Empty(restClient.HttpClient.DefaultRequestHeaders.UserAgent);
+    }
+
+    [Fact]
+    public void ConfigureDefaultParameters_sets_user_agent_given_httpClient_instance() {
         // arrange
         var httpClient    = new HttpClient();
         var clientOptions = new RestClientOptions();
 
         // act
-        var unused = new RestClient(httpClient, clientOptions);
-        var dummy  = new RestClient(httpClient, clientOptions);
+        var restClient = new RestClient(httpClient, clientOptions);
 
-        // assert
-        Assert.Contains(
-            httpClient.DefaultRequestHeaders.UserAgent,
-            agent => $"{agent.Product.Name}/{agent.Product.Version}" == clientOptions.UserAgent
-        );
-        Assert.Single(httpClient.DefaultRequestHeaders.UserAgent);
+        //assert
+        Assert.Single(
+            restClient.DefaultParameters,
+            parameter => parameter.Type == ParameterType.HttpHeader &&
+                parameter.Name == KnownHeaders.UserAgent &&
+                parameter.Value is string valueAsString &&
+                valueAsString == clientOptions.UserAgent);
+
+        Assert.Empty(httpClient.DefaultRequestHeaders.UserAgent);
     }
 }