fix double registration

src/InternshipSystem.Api/Controllers/RegistrationController.cs

@@ -47,8 +47,15 @@ namespace InternshipSystem.Api.Controllers
 
             var student = await _context.Students.FindAsync(user.PersonNumber);
 
+            try
+            {
                 edition.RegisterInternship(student);
                 await _context.SaveChangesAsync(token);
+            }
+            catch (ArgumentException e)
+            {
+                return BadRequest(e.Message);
+            }
 
             return Ok();
         }

src/InternshipSystem.Core/Entity/Edition.cs

@@ -31,6 +31,11 @@ namespace InternshipSystem.Core
 
         public void RegisterInternship(Student student)
         {
+            if (Internships.Any(i => i.Student.Id == student.Id))
+            {
+                throw new ArgumentException("error.registration.already_registered");
+            }
+            
             var internship = Internship.CreateStudentsInternship(student);
             
             Internships.Add(internship);