fix installation form behaviour if run multiple times in a single load

scripts.js

@@ -1,5 +1,10 @@
 // Convert each line of the textarea to a unique input element, so they're all passed with separate GET parameters
 function updateForm() {
+	// Delete old input elements in case the page hasn't been reloaded since last call
+	for (const oldInput of document.querySelectorAll('input[type="hidden"]')) {
+		oldInput.remove();
+	}
+
 	form = document.getElementById("install-form");
 	instances = document.getElementById("instance-input").value.split("\n");