Fixes the button duplication error in the map load action javascript.

js/map.action.load-map.js

+// Store an array of Backlinks globally and prevent clobbering existing ones.
+window.simBackLinks = window.simBackLinks || [];
+
 Drupal.simple_interactive_maps.ajax_load_map = function (elem, config, region) {
 
-  const backLinks = [];
   // locate the parent map element (with class .interactive_map__wrapper)
   const map = jQuery(elem).closest('.interactive_map__wrapper');
 
@@ -13,23 +15,26 @@ Drupal.simple_interactive_maps.ajax_load_map = function (elem, config, region) {
   const targetUrl = '/simple-interactive-maps/ajax';
 
   function buildBackLinks() {
-    if (backLinks.length > 0) {
+    let backLink = document.querySelector('.interactive_map__back-link');
+
+    if (!backLink && window.simBackLinks.length > 0) {
 
-      // Insert a link to load the previous map and insert it above the new map.
-      const backLink = document.createElement('a');
+      backLink = document.createElement('a');
       backLink.classList.add('interactive_map__back-link');
       backLink.classList.add('button');
       backLink.href = '#';
       backLink.innerHTML = 'Back to previous map';
+      mapParent.prepend(backLink);
+
       backLink.addEventListener('click', function (e) {
         e.preventDefault();
 
-        const linkConfig = backLinks.pop();
+        const linkConfig = window.simBackLinks.pop();
+
+        if (window.simBackLinks.length === 0) {
+          backLink.remove();
+        }
 
-        const backLinkElems = document.querySelectorAll('.interactive_map__back-link');
-        backLinkElems.forEach((backLinkElem) => {
-          backLinkElem.remove();
-        });
 
         const ajaxSettings = {
           url: targetUrl,
@@ -40,9 +45,8 @@ Drupal.simple_interactive_maps.ajax_load_map = function (elem, config, region) {
           buildBackLinks();
         });
       });
-
-      mapParent.prepend(backLink);
     }
+
   }
 
   elem.addEventListener('click', function (e) {
@@ -56,7 +60,7 @@ Drupal.simple_interactive_maps.ajax_load_map = function (elem, config, region) {
       },
     };
 
-    backLinks.push({
+    window.simBackLinks.push({
       map_to_load: map_id,
       target_element: config['target_map_id'],
     });