Fix duplicate compilation constants

Author: dsplaisted

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.BeforeCommon.targets

@@ -226,7 +226,7 @@ Copyright (c) .NET Foundation. All rights reserved.
           Condition=" '$(DisableImplicitFrameworkDefines)' != 'true' and '$(TargetPlatformIdentifier)' != '' and '$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), 5.0)) " >
     <ItemGroup>
       <_SupportedPlatformCompatibleVersions Include="@(SdkSupportedTargetPlatformVersion)" Condition=" %(Identity) != '' and $([MSBuild]::VersionLessThanOrEquals(%(Identity), $(TargetPlatformVersion))) " />
-      <_ImplicitDefineConstant Include="@(_SupportedPlatformCompatibleVersions->'$(TargetPlatformIdentifier.ToUpper())%(Identity)_OR_GREATER'->Replace('.', '_'))" />
+      <_ImplicitDefineConstant Include="@(_SupportedPlatformCompatibleVersions->Distinct()->'$(TargetPlatformIdentifier.ToUpper())%(Identity)_OR_GREATER'->Replace('.', '_'))" />
     </ItemGroup>
   </Target>