added new test case for saveStation & changed saveStation() to not saving duplicate stations

src/main/java/device/radioPlayer/RadioPlayer.java

@@ -183,7 +183,9 @@ public class RadioPlayer implements Device {
     }
 
     public String saveStation() {
+        if (savedPlaylist.contains(playedStation)) return "Station " + playedStation + " is already saved";
+        else
             savedPlaylist.add(playedStation);
-        return "";
+        return "Station " + playedStation + " is saved in your Station list";
     }
 }

src/test/java/device/radioPlayer/RadioPlayerTest.java

@@ -194,11 +194,14 @@ class RadioPlayerTest {
 
     static Stream<Arguments> saveStationOptions() {
         RadioPlayer rp1 = new RadioPlayer();
+        RadioPlayer rp2 = new RadioPlayer();
         rp1.changeRegion("BY");
         rp1.saveStation();
+        rp2.saveStation();
 
         return Stream.of(
-                Arguments.of("Test for saving station in saved playlist", rp1, "Antenne Bayern")
+                Arguments.of("Test for saving station in saved playlist", rp1, "Antenne Bayern"),
+                Arguments.of("Test for not saving station which is already in saved playlist", rp2, "MegaHits")
         );
     }