Make Windows PATH install idempotent

install.ps1

@@ -299,6 +299,26 @@ function Assert-SafeUpdateCache($Path) {
     $full
 }
 
+function Add-UserPath($PathToAdd) {
+    $userPath = [Environment]::GetEnvironmentVariable("Path", "User")
+    $entries = @()
+    if ($userPath) {
+        $entries = @($userPath -split ';' | Where-Object { $_ })
+    }
+    $wanted = Normalize-PathForCompare $PathToAdd
+    foreach ($entry in $entries) {
+        if ((Normalize-PathForCompare $entry) -ieq $wanted) {
+            return
+        }
+    }
+    $next = if ($entries.Count -gt 0) {
+        (@($entries) + $PathToAdd) -join ';'
+    } else {
+        $PathToAdd
+    }
+    [Environment]::SetEnvironmentVariable("Path", $next, "User")
+}
+
 $bindir = Join-Path $Prefix "bin"
 $configDir = Join-Path $HOME ".config\dosh"
 New-Item -ItemType Directory -Force -Path $bindir, $configDir | Out-Null
@@ -447,10 +467,7 @@ predict = true
 "@ | Set-Content -NoNewline -Encoding utf8 $hostsConfig
 }
 
-$userPath = [Environment]::GetEnvironmentVariable("Path", "User")
-if (-not (($userPath -split ';') -contains $bindir)) {
-    [Environment]::SetEnvironmentVariable("Path", "$userPath;$bindir", "User")
-}
+Add-UserPath $bindir
 
 Write-Host "Installed Dosh client to $bindir"
 Write-Host "Configured UDP port $Port"