added check for duplicate time-stamped GPS messages.

ArduCopter/ArduCopter.pde

@@ -869,7 +869,8 @@ static int16_t			superslow_loopCounter;
 static uint32_t 		loiter_timer;
 // disarms the copter while in Acro or Stabilize mode after 30 seconds of no flight
 static uint8_t 			auto_disarming_counter;
-
+// prevents duplicate GPS messages from entering system
+static uint32_t			last_gps_time;
 
 // Tracks if GPS is enabled based on statup routine
 // If we do not detect GPS at startup, we stop trying and assume GPS is not connected
@@ -1351,17 +1352,22 @@ static void update_GPS(void)
 	}
 
     if (g_gps->new_data && g_gps->fix) {
-
 		// clear new data flag
 		g_gps->new_data = false;
 
+    	// check for duiplicate GPS messages
+		if(last_gps_time != g_gps->time){
+
+	    	// look for broken GPS
+			// ---------------
 			gps_watchdog = 0;
 
 			// OK to run the nav routines
+			// ---------------
 			nav_ok = true;
 
-		// for performance
+			// for performance monitoring
-		// ---------------
+			// --------------------------
 			gps_fix_count++;
 
 			// used to calculate speed in X and Y, iterms
@@ -1409,6 +1415,10 @@ static void update_GPS(void)
 				alt_sensor_flag = true;
 			#endif
 		}
+
+		// save GPS time so we don't get duplicate reads
+		last_gps_time = g_gps->time;
+	}
 }
 
 void update_yaw_mode(void)