diff --git a/backend/src/services/students.ts b/backend/src/services/students.ts
index dc40e468..a8300db5 100644
--- a/backend/src/services/students.ts
+++ b/backend/src/services/students.ts
@@ -23,6 +23,8 @@ import { GroupDTO } from '@dwengo-1/common/interfaces/group';
 import { SubmissionDTO, SubmissionDTOId } from '@dwengo-1/common/interfaces/submission';
 import { QuestionDTO, QuestionId } from '@dwengo-1/common/interfaces/question';
 import { ClassJoinRequestDTO } from '@dwengo-1/common/interfaces/class-join-request';
+import {BadRequestException} from "../exceptions/bad-request-exception";
+import {ConflictException} from "../exceptions/conflict-exception";
 
 export async function getAllStudents(full: boolean): Promise<StudentDTO[] | string[]> {
     const studentRepository = getStudentRepository();
@@ -135,6 +137,10 @@ export async function createClassJoinRequest(username: string, classId: string):
     const student = await fetchStudent(username); // Throws error if student not found
     const cls = await fetchClass(classId);
 
+    if (cls.students.contains(student)) {
+        throw new ConflictException("Student already in this class");
+    }
+
     const request = mapToStudentRequest(student, cls);
     await requestRepo.save(request, { preventOverwrite: true });
     return mapToStudentRequestDTO(request);