diff --git a/src/main/java/catering/users/UserController.java b/src/main/java/catering/users/UserController.java
index f44cff1..9c22567 100644
--- a/src/main/java/catering/users/UserController.java
+++ b/src/main/java/catering/users/UserController.java
@@ -60,6 +60,12 @@ public class UserController {
 
 	@PostMapping("/register")
 	String register(@Valid RegistrationForm form, Errors result) {
+		if (
+			(!result.hasErrors()) &&
+			(!userManagement.getUserByName(form.getUsername()).isEmpty())
+		) {
+			result.rejectValue("username", "username already in use");
+		}
 		if (result.hasErrors()){
 			return "register";
 		}
@@ -85,6 +91,13 @@ public class UserController {
 	public String editProfile(@LoggedIn UserAccount userAccount, @Valid ProfileForm form, Errors result, Model model) {
 		String redirect = "redirect:/logout";
 
+		if (
+			(!result.hasErrors()) &&
+			(!userManagement.getUserByName(form.getUsername()).isEmpty()) &&
+			(userManagement.getUserByAccount(userAccount).get().getUsername() != form.getUsername())
+		) {
+			result.rejectValue("username", "username already in use");
+		}
 		if (result.hasErrors()){
 			return "profile";
 		}
@@ -138,6 +151,13 @@ public class UserController {
 	@PostMapping("/customers/edit/{id}")
 	@PreAuthorize("hasRole('ADMIN')")
 	public String updateCustomer(@PathVariable("id") User user, @Valid AdminForm form, Errors result) {
+		if (
+			!result.hasErrors() &&
+			!userManagement.getUserByName(form.getUsername()).isEmpty() &&
+			!user.getUsername().equals(form.getUsername())
+		) {
+			result.rejectValue("username", "username already in use");
+		}
 		if (result.hasErrors()){
 			return "edit-customer";
 		}
diff --git a/src/test/java/catering/users/UserControllerIntegrationTests.java b/src/test/java/catering/users/UserControllerIntegrationTests.java
index 69f436f..a6456b6 100644
--- a/src/test/java/catering/users/UserControllerIntegrationTests.java
+++ b/src/test/java/catering/users/UserControllerIntegrationTests.java
@@ -57,6 +57,8 @@ class UserControllerIntegrationTests {
 		createTestUser("jacob");
 		createTestUser("moana");
 		createTestUser("paul");
+		createTestUser("mark");
+		createTestUser("eva");
 	}
 
 	@Test
@@ -308,4 +310,47 @@ class UserControllerIntegrationTests {
 				.extracting("username")
 				.contains("jacob");
 	}
+
+	@Test
+	@WithAnonymousUser
+	void userAlreadyExistsRegistration() throws Exception {
+		MockHttpServletRequestBuilder createCustomer = post("/register")
+				.param("username", "admin")
+				.param("password", "123")
+				.param("fullName", "Tom Klaus")
+				.param("address", "Baum Weg");
+		mvc.perform(createCustomer).andExpect(content().string(containsString("Ungültiger Nutzername")));
+	}
+
+	@Test
+	@WithMockUser(username = "mark", password = "123")
+	void changeUsernameToExistingUsername() throws Exception {
+		assertTrue(userAccountManagement.findByUsername("jacob").isPresent());
+		assertTrue(userAccountManagement.findByUsername("mark").isPresent());
+		MockHttpServletRequestBuilder createCustomer = post("/profile")
+				.param("username", "admin")
+				.param("password", "")
+				.param("fullName", "Mark Klaus")
+				.param("address", "Baum Weg");
+		mvc.perform(createCustomer).andExpect(content().string(containsString("Ungültiger Nutzername")));
+		createCustomer = post("/profile")
+				.param("username", "jacob")
+				.param("password", "")
+				.param("fullName", "Mark Klaus")
+				.param("address", "Baum Weg");
+		mvc.perform(createCustomer).andExpect(content().string(containsString("Ungültiger Nutzername")));
+		assertTrue(userAccountManagement.findByUsername("mark").isPresent());
+	}
+
+	@Test
+	@WithMockUser(username = "admin", password = "admin", roles = "ADMIN")
+	void changeCustomerUsernameToExistingUsername() throws Exception {
+		assertTrue(userAccountManagement.findByUsername("eva").isPresent());
+		mvc.perform(post("/customers/edit/" + userManagement.getUserByName("eva").get().getId())
+			.param("username", "admin")
+			.param("fullName", "Eva Klaus")
+			.param("address", "Baum Weg")
+			).andExpect(content().string(containsString("Ungültiger Nutzername")));
+		assertTrue(userAccountManagement.findByUsername("eva").isPresent());
+	}
 }