Don't duplicate the messages from logcat

Fixes NativeScript/nativescript-cli#1130

mobile/android/logcat-helper.ts

@@ -3,14 +3,18 @@
 import byline = require("byline");
 
 export class LogcatHelper implements Mobile.ILogcatHelper {
+	private mapDeviceToLoggingStarted: IDictionary<boolean>;
+
 	constructor(private $childProcess: IChildProcess,
 			private $deviceLogProvider: Mobile.IDeviceLogProvider,
 			private $devicePlatformsConstants: Mobile.IDevicePlatformsConstants,
 			private $logger: ILogger,
-			private $staticConfig: Config.IStaticConfig) { }
+			private $staticConfig: Config.IStaticConfig) {
+				this.mapDeviceToLoggingStarted = Object.create(null);
+			}
 
 	public start(deviceIdentifier: string): void {
-		if(deviceIdentifier) {
+		if (deviceIdentifier && !this.mapDeviceToLoggingStarted[deviceIdentifier]) {
 			let adbPath = this.$staticConfig.getAdbFilePath().wait();
 			// remove cached logs:
 			this.$childProcess.spawnFromEvent(adbPath, ["-s", deviceIdentifier,  "logcat",  "-c"], "close",  {}, {throwError: false}).wait();
@@ -31,6 +35,8 @@ export class LogcatHelper implements Mobile.ILogcatHelper {
 				let lineText = line.toString();
 				this.$deviceLogProvider.logData(lineText, this.$devicePlatformsConstants.Android, deviceIdentifier);
 			});
+
+			this.mapDeviceToLoggingStarted[deviceIdentifier] = true;
 		}
 	}
 }