Fix bug of adding duplicate freq to ground station's freq list

src/CommNetConstellation/UI/GroundStationEditDialog.cs

@@ -143,7 +143,7 @@ private void addClick()
                     {
                         throw new Exception("Frequency cannot be negative");
                     }
-                    else if (this.hostStation.Frequencies.Contains(newFreq))
+                    else if (this.freqListShown.Contains(newFreq))
                     {
                         throw new Exception("Ground station has this frequency already");
                     }