This seems related:
Double Key Code.
If what you are trying to achieve is using the Shift Key and the C key to do the button15.PerformClick() then you have to check the Modifiers in the KeyEventArgs instance.
Try this:
private void Main_Form_KeyDown(object sender, KeyEventArgs e)
{
if (e.KeyCode == Keys.C && e.Modifiers == Keys.Shift)
{
MessageBox.Show("Enter Key Pressed ");
button15.PerformClick();
}
else if (e.KeyCode == Keys.K)
{
MessageBox.Show("Enter Key Pressed ");
button16.PerformClick();
}
}