I've the same issue in .net9. Depending on when and where I try to create the service, it will either work or not.

For example if I create the service client in the Program.cs when the application starts, it will work, I can call WhoAmI api just fine.

But if I wait a bit, and try to create the service later on in the application lifecycle, it won't work and raise the below error with types.

The Current type and Existing type are exactly the same, so I don't know what is going on here.

    Microsoft.PowerPlatform.Dataverse.Client.Utils.DataverseConnectionException: Failed to Clone Connection
     ---> Microsoft.PowerPlatform.Dataverse.Client.Utils.DataverseOperationException: Failed constructing cloned connection. debugstate=1
     ---> Microsoft.PowerPlatform.Dataverse.Client.Utils.DataverseOperationException: Exception - Failed to lookup current user
     ---> System.ArgumentException: A proxy type with the name account has been defined by another assembly. 

Current type: Integrations.Dynamics365.Entities.Account, Integrations, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null, 
Existing type: Integrations.Dynamics365.Entities.Account, Integrations, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null (Parameter 'account')

       at Microsoft.PowerPlatform.Dataverse.Client.ServiceClient.Execute(OrganizationRequest request)