Make sure we do not display twice the same background layer in selector

At this stage, uMap does not distinguish between a custom background
and the default background, both are saved in map.options.tilelayer.

Given we want a custom background (so not in the list) to appear in
the selector, we need this check to be sure we are not adding again
one layer from the list

umap/static/umap/js/umap.js

@@ -651,8 +651,18 @@ L.U.Map.include({
   },
 
   eachTileLayer: function (callback, context) {
-    if (this.customTilelayer) callback.call(context, this.customTilelayer)
+    const urls = []
-    this.tilelayers.forEach((layer) => callback.call(context, layer))
+    const callOne = (layer) => {
+      // Prevent adding a duplicate background,
+      // while adding selected/custom on top of the list
+      const url = layer.options.url_template
+      if (urls.indexOf(url) !== -1) return
+      callback.call(context, layer)
+      urls.push(url)
+    }
+    if (this.selected_tilelayer) callOne(this.selected_tilelayer)
+    if (this.customTilelayer) callOne(this.customTilelayer)
+    this.tilelayers.forEach(callOne)
   },
 
   setOverlay: function () {