I have similar problem. I have removed visual studio a few times and  install again following all steps on the axi doc. I allways get the same error. I can install all other plugins so I guess is not a visual studio problem. Using latest unreal for aximmetry and ue4.26.1 and latest visualstudio community 2019 Any help will be appreciated.

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSS\DLSS.Build.cs(23,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSS'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSSBlueprint\DLSSBlueprint.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSSBlueprint'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSSEditor\DLSSEditor.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSSEditor'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\DLSSUtility\DLSSUtility.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'DLSSUtility'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXD3D11RHI\NGXD3D11RHI.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXD3D11RHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXD3D12RHI\NGXD3D12RHI.Build.cs(23,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXD3D12RHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXRHI\NGXRHI.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXRHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXVulkanRHI\NGXVulkanRHI.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXVulkanRHI'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\NGXVulkanRHIPreInit\NGXVulkanRHIPreInit.Build.cs(24,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGXVulkanRHIPreInit'

LogAximmetry: Display: Plugin installer: c:\UnrealEngine\UE_4.26\Engine\Plugins\Runtime\Nvidia\DLSS\Source\ThirdParty\NGX\NGX.Build.cs(25,14) : error CS0101: The namespace '<global namespace>' already contains a definition for 'NGX'

LogAximmetry: Display: Plugin installer: ERROR: Unable to compile source files.

LogAximmetry: Error: Plugin installer: Error: DLSS could not be built. UnrealBuildTool exited with 6.