In my opening form I am using an MdiParent but the ControlBox is showing twice (see image). I set the ControlBox's property to false in all the child forms. But it didn't change. How could I close the control boxes?

In my opening form I am using an MdiParent but the ControlBox is showing twice (see image). I set the ControlBox's property to false in all the child forms. But it didn't change. How could I close the control boxes?
