fix: handle duplicate input

internal/services/auth/register_user.go

@@ -9,6 +9,12 @@ import (
 )
 
 func (a *AuthSvc) RegisterUser(spec authdomain.RegisterUserReq) (string, error) {
+	_, err := a.userRepo.GetUserByEmail(spec.Email)
+
+	if err == nil {
+		return "", errors.New("this email address is already in use")
+	}
+
 	subsId, err := a.subsRepo.Create(spec.SubscribePlanID)
 	if err != nil {
 		return "", nil