Fixing the tool used in the test that I changed when trying things out and fixing the code that sets environment variables in the command to check if the environment variable already exists before adding it.

TestAssets/TestProjects/MSBuildTestAppWithToolInDependencies/MSBuildTestAppWithToolInDependencies.csproj

@@ -27,7 +27,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <DotNetCliToolReference Include="dotnet-dependency-tool-invoker">
+    <DotNetCliToolReference Include="dotnet-invoke-portable">
       <Version>1.0.0-rc-*</Version>
     </DotNetCliToolReference>
   </ItemGroup>

src/Microsoft.DotNet.Cli.Utils/Command.cs

@@ -31,7 +31,10 @@ namespace Microsoft.DotNet.Cli.Utils
 
             foreach(var environmentVariable in commandSpec.EnvironmentVariables)
             {
-                psi.Environment.Add(environmentVariable);
+                if (!psi.Environment.ContainsKey(environmentVariable.Key))
+                {
+                    psi.Environment.Add(environmentVariable.Key, environmentVariable.Value);
+                }
             }
 
             _process = new Process