No, there isn’t a way to stop this. The way that ACF works does not have the ability to have options groups and it does not allow multiple duplicate values. Why? ACF stores the choices your enter in an array that looks something like this:
$field = array(
// other field settings
'choices' => array(
'value 1' => 'label 1',
'value 2' => 'label 2',
// etc...
),
// other field settings
)
The problem here is that PHP does not allow duplicate array index values, which is where your choice value lives.
