Reopening a form ?

i have a main form which has 2 login button.One of them is for duty and the other is for student. Once a clicked on one of them the related form is opened. But when i closed the related form and then i wanted to open the form again but it can not work. How can i fix it?

it is the code which i can open the student form

Code:

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StudentForm = new frmStudent();
StudentForm.MainForm = this;
StudentForm.Show();

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call the .Show() method again. Declare your form in your classes scope so that way you can call it from any of its methods and it will be the same instance of the form (IE any data members on that form will still exist).