Install same emitter only once when generating SDK

eng/common/scripts/TypeSpec-Project-Generate.ps1

@@ -14,6 +14,87 @@ $ErrorActionPreference = "Stop"
 . $PSScriptRoot/common.ps1
 Install-ModuleIfNotInstalled "powershell-yaml" "0.4.1" | Import-Module
 
+function GetNpmPackageVersion([string]$packagePath) {
+    $packagePath = Resolve-Path $packagePath
+    $packageFolder = Split-Path -Parent $packagePath
+    $packageName = Split-Path -Leaf $packagePath
+    Push-Location $packageFolder
+    try {
+        $packageVersion = node -p -e "require('./$packageName').version"
+        return $packageVersion
+    }
+    finally {
+        Pop-Location
+    }
+}
+
+function GetEmitterDependencyVersion([string]$packagePath) {
+    $matchString = "`"$emitterName`": `"(.*?)`""
+    If ((Get-Content -Raw $packagePath) -match $matchString) {
+        return $Matches[1]
+    }
+}
+
+function NpmInstallAtRoot() {
+    $root = Resolve-Path "$PSScriptRoot/../../.."
+    Push-Location $root
+    try {
+        #default to root/eng/emitter-package.json but you can override by writing
+        #Get-${Language}-EmitterPackageJsonPath in your Language-Settings.ps1
+        $replacementPackageJson = "$PSScriptRoot/../../emitter-package.json"
+        if (Test-Path "Function:$GetEmitterPackageJsonPathFn") {
+            $replacementPackageJson = &$GetEmitterPackageJsonPathFn
+        }
+
+        $replacementVersion = GetEmitterDependencyVersion $replacementPackageJson
+        Write-Host "Replacement emitter version: $replacementVersion"
+
+        $installedPath = Join-Path $root "node_modules" $emitterName "package.json"
+        Write-Host "Checking installed emitter at $installedPath"
+        if (Test-Path $installedPath) {
+            $installedVersion = GetNpmPackageVersion $installedPath
+            Write-Host "Installed emitter version: $installedVersion"
+
+            if ($installedVersion -eq $replacementVersion) {
+                Write-Host "Emitter already installed. Skip installing."
+                return
+            }
+        }
+
+        Write-Host "Installing package at"(Get-Location)
+
+        if (Test-Path "node_modules") {
+            Write-Host "Remove existing node_modules at"(Get-Location)
+            Remove-Item -Path "node_modules" -Force -Recurse
+        }
+
+        $useAlphaNpmRegistry = (Get-Content $replacementPackageJson -Raw).Contains("-alpha.")
+        if($useAlphaNpmRegistry) {
+            Write-Host "Package.json contains '-alpha.' in the version, Creating .npmrc using public/azure-sdk-for-js-test-autorest feed."
+            "registry=https://pkgs.dev.azure.com/azure-sdk/public/_packaging/azure-sdk-for-js-test-autorest/npm/registry/ `n`nalways-auth=true" | Out-File '.npmrc'
+        }
+
+        $hasPackageFile = Test-Path "package.json"
+
+        $npmInstallCommand = "npm install --prefix $root $emitterName@$replacementVersion --no-package-lock --omit=dev"
+        Write-Host($npmInstallCommand)
+        Invoke-Expression $npmInstallCommand
+        if ($LASTEXITCODE) { exit $LASTEXITCODE }
+
+        if ($hasPackageFile) {
+            git restore package.json
+        }
+        else {
+            rm package.json
+        }
+
+        if ($LASTEXITCODE) { exit $LASTEXITCODE }
+    }
+    finally {
+        Pop-Location
+    }
+}
+
 function NpmInstallForProject([string]$workingDirectory) {
     Push-Location $workingDirectory
     try {
@@ -36,25 +117,7 @@ function NpmInstallForProject([string]$workingDirectory) {
             Remove-Item -Path "package-lock.json" -Force
         }
 
-        #default to root/eng/emitter-package.json but you can override by writing
-        #Get-${Language}-EmitterPackageJsonPath in your Language-Settings.ps1
-        $replacementPackageJson = Join-Path $PSScriptRoot "../../emitter-package.json"
-        if (Test-Path "Function:$GetEmitterPackageJsonPathFn") {
-            $replacementPackageJson = &$GetEmitterPackageJsonPathFn
-        }
-
-        Write-Host("Copying package.json from $replacementPackageJson")
-        Copy-Item -Path $replacementPackageJson -Destination "package.json" -Force
-
-        $useAlphaNpmRegistry = (Get-Content $replacementPackageJson -Raw).Contains("-alpha.")
-
-        if($useAlphaNpmRegistry) {
-            Write-Host "Package.json contains '-alpha.' in the version, Creating .npmrc using public/azure-sdk-for-js-test-autorest feed."
-            "registry=https://pkgs.dev.azure.com/azure-sdk/public/_packaging/azure-sdk-for-js-test-autorest/npm/registry/ `n`nalways-auth=true" | Out-File '.npmrc'
-        }
-
-        npm install --no-lock-file
-        if ($LASTEXITCODE) { exit $LASTEXITCODE }
+        NpmInstallAtRoot
     }
     finally {
         Pop-Location