feat(ToastContainer): add prevents duplicates functionality

* Thanks to @RangarajKaushikSundar 
* Closes #60

package.json

@@ -85,6 +85,7 @@
   "dependencies": {
     "classnames": "^2.2.3",
     "element-class": "^0.2.2",
+    "lodash": "^4.13.1",
     "react-addons-update": "^0.14.0 || ^15.0.0",
     "react-dom": "^0.14.0 || ^15.0.0"
   }

src/ToastContainer.js

@@ -12,6 +12,8 @@ import {
   default as ToastMessage,
 } from "./ToastMessage";
 
+import _ from "lodash";
+
 export default class ToastContainer extends Component {
 
   static propTypes = {
@@ -45,7 +47,7 @@ export default class ToastContainer extends Component {
   state = {
     toasts: [],
     toastId: 0,
-    previousMessage: null,
+    messageList: [],
   };
 
   error(message, title, optionsOverride) {
@@ -72,7 +74,7 @@ export default class ToastContainer extends Component {
 
   _notify(type, message, title, optionsOverride = {}) {
     if (this.props.preventDuplicates) {
-      if (this.state.previousMessage === message) {
+      if (_.includes(this.state.messageList, message)) {
         return;
       }
     }
@@ -99,9 +101,13 @@ export default class ToastContainer extends Component {
       [`${this.props.newestOnTop ? `$unshift` : `$push`}`]: [newToast],
     };
 
+    const messageOperation = {
+      [`${this.props.newestOnTop ? `$unshift` : `$push`}`]: [message],
+    };
+
     const nextState = update(this.state, {
       toasts: toastOperation,
-      previousMessage: { $set: message },
+      messageList: messageOperation,
     });
     this.setState(nextState);
   }
@@ -126,6 +132,7 @@ export default class ToastContainer extends Component {
       }
       this.setState(update(this.state, {
         toasts: { $splice: [[index, 1]] },
+        messageList: { $splice: [[index, 1]] },
       }));
       return true;
     }, false);