AP_WindVane_NMEA: decode each sentence once

libraries/AP_WindVane/AP_WindVane_NMEA.cpp

@@ -86,6 +86,10 @@ bool AP_WindVane_NMEA::decode(char c)
     case '\n':
     case '*':
     {
+        if (_sentence_done) {
+            return false;
+        }
+
         // null terminate and decode latest term
         _term[_term_offset] = 0;
         bool valid_sentence = decode_latest_term();
@@ -105,6 +109,7 @@ bool AP_WindVane_NMEA::decode(char c)
         _term_is_checksum = false;
         _wind_dir_deg = -1.0f;
         _speed_ms = -1.0f;
+        _sentence_done = false;
         return false;
     }
 
@@ -125,6 +130,7 @@ bool AP_WindVane_NMEA::decode_latest_term()
 {
     // handle the last term in a message
     if (_term_is_checksum) {
+        _sentence_done = true;
         uint8_t checksum = 16 * char_to_hex(_term[0]) + char_to_hex(_term[1]);
         return ((checksum == _checksum) && _sentence_valid);
     }

libraries/AP_WindVane/AP_WindVane_NMEA.h

@@ -55,4 +55,5 @@ private:
     uint8_t _checksum;         // checksum accumulator
     bool _term_is_checksum;    // current term is the checksum
     bool _sentence_valid;      // is current sentence valid so far
+    bool _sentence_done;       // true if this sentence has already been decoded
 };