Added redundant plugin load validation.

Signed-off-by: TheDudeFromCI <thedudefromci@gmail.com>

docs/api.md

@@ -1369,7 +1369,8 @@ See the `bot.settings` property.
 
 #### bot.loadPlugin(plugin)
 
-Injects a Plugin.
+Injects a Plugin. Does nothing if the plugin is already loaded.
+
  * `plugin` - function
 
 ```js
@@ -1392,6 +1393,10 @@ bot.once('login', function() {
 Injects plugins see `bot.loadPlugin`.
  * `plugins` - array of functions
 
+#### bot.hasPlugin(plugin)
+
+Checks if the given plugin is loaded (or scheduled to be loaded) on this bot.
+
 #### bot.sleep(bedBlock, [cb])
 
 Sleep in a bed. `bedBlock` should be a `Block` instance which is a bed. `cb` can have an err parameter if the bot cannot sleep.

index.d.ts

@@ -225,6 +225,8 @@ export class Bot extends (EventEmitter as new () => TypedEmitter<BotEvents>) {
 
   loadPlugins(plugins: Array<Plugin>): void;
 
+  hasPlugin(plugin: Plugin): boolean;
+
   sleep(bedBlock: Block, cb?: (err?: Error) => void): void;
 
   isABed(bedBlock: Block): void;

lib/plugin_loader.js

@@ -4,29 +4,49 @@ module.exports = inject
 
 function inject (bot, options) {
   let loaded = false
-  let pluginsToBeAdded = []
+  let pluginList = []
   bot.once('inject_allowed', onInjectAllowed)
 
   function onInjectAllowed () {
     loaded = true
-    injectPlugins(pluginsToBeAdded)
+    injectPlugins()
   }
+
   function loadPlugin (plugin) {
     assert.ok(typeof plugin === 'function', 'plugin needs to be a function')
-    if (!loaded) pluginsToBeAdded.push(plugin)
-    else plugin(bot, options)
+
+    if (hasPlugin(plugin)) {
+      return
+    }
+
+    pluginList.push(plugin)
+
+    if (loaded) {
+      plugin(bot, options)
+    }
   }
+
   function loadPlugins (plugins) {
+    // While type checking if already done in the other function, it's useful to do
+    // it here to prevent situations where only half the plugin list is loaded.
     assert.ok(plugins.filter(plugin => typeof plugin === 'function').length === plugins.length, 'plugins need to be an array of functions')
-    if (loaded) return injectPlugins(plugins)
-    pluginsToBeAdded = pluginsToBeAdded.concat(plugins)
-  }
-  function injectPlugins (plugins) {
+
     plugins.forEach((plugin) => {
+      loadPlugin(plugin)
+    })
+  }
+
+  function injectPlugins () {
+    pluginList.forEach((plugin) => {
       plugin(bot, options)
     })
   }
 
+  function hasPlugin (plugin) {
+    return pluginList.indexOf(plugin) >= 0
+  }
+
   bot.loadPlugin = loadPlugin
   bot.loadPlugins = loadPlugins
+  bot.hasPlugin = hasPlugin
 }