Create The Children Form Within A Same MDI Form Every Time?

i MDI form there, i was create the children form within a same MDI form every time i click on the button to show the form, which means i create a new form, how i do for ensure that each time i click on the button there will not create a new form is the form is really exist? how to avoid duplicate of the form will not exist twice?another 1 more question is also about the MDI form, everytime i create i new form, the new form is covered by old form, how i do that the new form i created will always show infront, other 1 will show at behide