Cannot update traffic manager CustomHeader

[donma]

Query/Question

I set a custom header test "testheader:I_AM_STRING" on the Azure portal.
I added new custom header is no problem, but I tried to update the header value to "app_edited".

            var trafficManagerProfile = azure.TrafficManagerProfiles.GetById("/subscriptions/xxxxxxxx-dddd-xxxx-ssss-ddddddddddddd/resourceGroups/spinus/providers/Microsoft.Network/trafficManagerProfiles/site1");

            var target = trafficManagerProfile.Update().UpdateExternalTargetEndpoint("client1")
                .WithCustomHeader("testheader", "app_edited").Parent().Apply();

I got error message

Unhandled exception. System.AggregateException: One or more errors occurred. (Duplicate custom header names for app_edited are not allowed.)
 ---> Microsoft.Rest.Azure.CloudException: Duplicate custom header names for app_edited are not allowed.
   at Microsoft.Azure.Management.TrafficManager.Fluent.EndpointsOperations.CreateOrUpdateWithHttpMessagesAsync(String resourceGroupName, String profileName, String endpointType, String endpointName, EndpointInner parameters, Dictionary`2 customHeaders, CancellationToken cancellationToken)
   at Microsoft.Azure.Management.TrafficManager.Fluent.EndpointsOperationsExtensions.CreateOrUpdateAsync(IEndpointsOperations operations, String resourceGroupName, String profileName, String endpointType, String endpointName, EndpointInner parameters, CancellationToken cancellationToken)
   at Microsoft.Azure.Management.TrafficManager.Fluent.TrafficManagerEndpointImpl.CreateAsync(CancellationToken cancellationToken)
   at Microsoft.Azure.Management.TrafficManager.Fluent.TrafficManagerEndpointImpl.UpdateAsync(CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at Microsoft.Azure.Management.TrafficManager.Fluent.TrafficManagerProfileImpl.UpdateResourceAsync(CancellationToken cancellationToken)
   at Microsoft.Azure.Management.TrafficManager.Fluent.TrafficManagerProfileImpl.CreateResourceAsync(CancellationToken cancellationToken)
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.ResourceActions.Creatable`4.Microsoft.Azure.Management.ResourceManager.Fluent.Core.ResourceActions.IResourceCreator<IResourceT>.CreateResourceAsync(CancellationToken cancellationToken)
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.DAG.CreatorTaskItem`1.ExecuteAsync(CancellationToken cancellationToken)
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.DAG.TaskGroupBase`1.ExecuteNodeTaskAsync(DAGNode`1 node, CancellationToken cancellationToken)
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.ResourceActions.CreatableUpdatable`5.ApplyAsync(CancellationToken cancellationToken, Boolean multiThreaded)
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.Extensions.Synchronize[TResult](Func`1 function)
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.ResourceActions.CreatableUpdatable`5.Apply()

Environment: .net core 3.1 + Microsoft.Azure.Management.Fluent 1.36.1

the error message on Azure portal

作業名稱
Create or Update a Traffic Manager External Endpoint
時間戳記
Tue Dec 22 2020 18:04:04 GMT+0800 (台北標準時間)
事件起始者
site1trafficmanager
錯誤碼
BadRequest
訊息
Duplicate custom header names for app_edited are not allowed.

thanks for help.