diff --git a/controllers/mentor.go b/controllers/mentor.go
index 769ad6a..92ac8d1 100644
--- a/controllers/mentor.go
+++ b/controllers/mentor.go
@@ -99,6 +99,29 @@ func RegisterMentor(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Check if a student of the same username exists
+	student := models.Student{}
+	tx = db.
+		Table("students").
+		Where("username = ?", reqFields.Username).
+		First(&student)
+	if tx.Error != nil && tx.Error != gorm.ErrRecordNotFound {
+		utils.LogErrAndRespond(r, w, tx.Error, "Database error.", http.StatusInternalServerError)
+		return
+	}
+	student_exists := student.Username == reqFields.Username
+
+	if student_exists {
+		utils.LogWarnAndRespond(
+			r,
+			w,
+			fmt.Sprintf("The username `%s` already exists as a student.", reqFields.Username),
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
 	// Create a db entry if the mentor doesn't exist
 	tx = db.Create(&models.Mentor{
 		Username: reqFields.Username,
diff --git a/controllers/mentor_test.go b/controllers/mentor_test.go
index 0eab895..a994485 100644
--- a/controllers/mentor_test.go
+++ b/controllers/mentor_test.go
@@ -100,6 +100,30 @@ func tMentorRegExistingUser(db *gorm.DB, t *testing.T) {
 	expectResponseJSONBodyToBe(t, res, utils.HTTPMessage{Code: http.StatusBadRequest, Message: fmt.Sprintf("Mentor `%s` already exists.", testUsername)})
 }
 
+// Test an existing student registration request to /mentor/form/ with proper authentication and input
+func tMentorRegAsStudent(db *gorm.DB, t *testing.T) {
+	// Test login fields
+	testUsername := getTestUsername()
+	testLoginFields := utils.LoginJwtFields{Username: testUsername}
+
+	testJwt, _ := utils.GenerateLoginJwtString(testLoginFields)
+	studentFields := controllers.RegisterStudentReqFields{Username: testUsername}
+
+	req := createStudentRegRequest(&studentFields)
+	req.Header.Add("Bearer", testJwt)
+
+	_ = executeRequest(req, db)
+
+	mentorFields := controllers.RegisterMentorReqFields{Username: testUsername}
+	req = createMentorRegRequest(&mentorFields)
+	req.Header.Add("Bearer", testJwt)
+
+	res := executeRequest(req, db)
+
+	expectStatusCodeToBe(t, res, http.StatusBadRequest)
+	expectResponseJSONBodyToBe(t, res, utils.HTTPMessage{Code: http.StatusBadRequest, Message: fmt.Sprintf("The username `%s` already exists as a student.", testUsername)})
+}
+
 // Test requests to /mentor/form/ with proper authentication and input
 func TestMentorRegOK(t *testing.T) {
 	// Set up a local test database path
@@ -125,6 +149,14 @@ func TestMentorRegOK(t *testing.T) {
 			tMentorRegExistingUser(db, t)
 		},
 	)
+
+	// Student registering as mentor test
+	t.Run(
+		"Test: Student registering as mentor.",
+		func(t *testing.T) {
+			tMentorRegAsStudent(db, t)
+		},
+	)
 }
 
 func createFetchMentorRequest() *http.Request {
diff --git a/controllers/student.go b/controllers/student.go
index 7240b6f..1651385 100644
--- a/controllers/student.go
+++ b/controllers/student.go
@@ -110,6 +110,29 @@ func RegisterStudent(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Check if a mentor of the same username exists
+	mentor := models.Mentor{}
+	tx = db.
+		Table("mentors").
+		Where("username = ?", reqFields.Username).
+		First(&mentor)
+	if tx.Error != nil && tx.Error != gorm.ErrRecordNotFound {
+		utils.LogErrAndRespond(r, w, tx.Error, "Database error.", http.StatusInternalServerError)
+		return
+	}
+	mentor_exists := mentor.Username == reqFields.Username
+
+	if mentor_exists {
+		utils.LogWarnAndRespond(
+			r,
+			w,
+			fmt.Sprintf("The username `%s` already exists as a mentor.", reqFields.Username),
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
 	// Create a db entry if the student doesn't exist
 	tx = db.Create(&models.Student{
 		Username: reqFields.Username,
diff --git a/controllers/student_test.go b/controllers/student_test.go
index 2509dc2..fb8794d 100644
--- a/controllers/student_test.go
+++ b/controllers/student_test.go
@@ -101,6 +101,30 @@ func tStudentRegExistingUser(db *gorm.DB, t *testing.T) {
 	expectResponseJSONBodyToBe(t, res, utils.HTTPMessage{Code: http.StatusBadRequest, Message: fmt.Sprintf("Student `%s` already exists.", testUsername)})
 }
 
+// Test an existing mentor registration request to /student/form/ with proper authentication and input
+func tStudentRegAsMentor(db *gorm.DB, t *testing.T) {
+	// Test login fields
+	testUsername := getTestUsername()
+	testLoginFields := utils.LoginJwtFields{Username: testUsername}
+
+	testJwt, _ := utils.GenerateLoginJwtString(testLoginFields)
+	mentorFields := controllers.RegisterMentorReqFields{Username: testUsername}
+
+	req := createMentorRegRequest(&mentorFields)
+	req.Header.Add("Bearer", testJwt)
+
+	_ = executeRequest(req, db)
+
+	studentsFields := controllers.RegisterStudentReqFields{Username: testUsername}
+	req = createStudentRegRequest(&studentsFields)
+	req.Header.Add("Bearer", testJwt)
+
+	res := executeRequest(req, db)
+
+	expectStatusCodeToBe(t, res, http.StatusBadRequest)
+	expectResponseJSONBodyToBe(t, res, utils.HTTPMessage{Code: http.StatusBadRequest, Message: fmt.Sprintf("The username `%s` already exists as a mentor.", testUsername)})
+}
+
 // Test requests to /student/form/ with proper authentication and input
 func TestStudentRegOK(t *testing.T) {
 	// Set up a local test database path
@@ -126,6 +150,14 @@ func TestStudentRegOK(t *testing.T) {
 			tStudentRegExistingUser(db, t)
 		},
 	)
+
+	// Mentor registering as student test
+	t.Run(
+		"Test: Mentor registering as student.",
+		func(t *testing.T) {
+			tStudentRegAsMentor(db, t)
+		},
+	)
 }
 
 func createStudentBlogLinkRequest(reqFields *controllers.StudentBlogLinkReqFields) *http.Request {