refactoring: fixed initializeDummyData in Main to only be able to run once

src/main/java/org/example/Main.java

@@ -51,9 +51,9 @@ public class Main {
                     if(!DummyDataExists){
                         administration.initializeDummyData();
                         System.out.println("Dummy data initialized.");
-
+                        DummyDataExists = true;
                     }
-                    else{System.out.println("Dummy Data could already exists");}
+                    else{System.out.println("Dummy Data already exists");}
                     break;
 
                 case 8: