added Tests and fix addUserEvent

src/main/java/mops/gruppen2/domain/Group.java

@@ -52,7 +52,9 @@ public class Group extends Aggregate {
     private void applyEvent(AddUserEvent event) {
         User user = new User(event.getUser_id(), event.getGivenname(), event.getFamilyname(), event.getEmail());
 
-        this.members.add(user);
+        if (!this.members.contains(user)){
+            this.members.add(user);
+        }
     }
 
     private void applyEvent(UpdateGroupTitleEvent event) {

src/main/java/mops/gruppen2/domain/User.java

@@ -2,12 +2,16 @@ package mops.gruppen2.domain;
 
 import lombok.AllArgsConstructor;
 import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.Value;
 
-@Data
+@Value
 @AllArgsConstructor
+@EqualsAndHashCode(exclude = {"givenname", "familyname", "email"})
 public class User {
 
     String user_id;
+
     String givenname;
     String familyname;
     String email;