Set the main activity as singTask so that when you press the notification multiple time, it starts it only once.

Explained here http://stackoverflow.com/a/29847821

app/src/main/AndroidManifest.xml

@@ -10,20 +10,23 @@
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
     <uses-permission android:name="android.permission.WAKE_LOCK" />
 
-    <application android:label="@string/app_name" android:icon="@drawable/ic_launcher" android:allowBackup="true" android:theme="@style/TraccarTheme">
+    <application
+        android:allowBackup="true"
+        android:icon="@drawable/ic_launcher"
+        android:label="@string/app_name"
+        android:theme="@style/TraccarTheme">
 
-        <activity android:name=".MainActivity">
+        <activity android:name=".MainActivity"
+            android:launchMode="singleTask">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
 
-        <activity android:name=".StatusActivity">
+        <activity android:name=".StatusActivity"/>
-        </activity>
 
-        <activity android:name=".AboutActivity">
+        <activity android:name=".AboutActivity"/>
-        </activity>
 
         <service android:name=".TrackingService" />