In the Samples of Version 10.2.7 it was possible to display an overview map. (arcgis-runtime-samples-dotnet-master\src\Desktop\ArcGISRuntimeSamplesDesktop\Samples\Mapping\OverviewMap.xaml)
How can I accomplish this with the 100.2.1 Version?
The system tells me "the object already owned" when I try to use the map again.
I'm sorry to hear you're running into problems.
There is a sample for 100.2.1 that has something similar to an overview map: arcgis-runtime-samples-dotnet/src/WPF/ArcGISRuntime.WPF.Viewer/Samples/GraphicsOverlay/Animate3DGrap...
You will need to use a separate map; this isn't called out specifically in the sample above because it uses a scene.
Let me know if you run into any other issues.