Suggestions is a PHP array that is provided by a hook and an alter hook. The standard rules apply, alter hook is later, so add it there to be more likely to be the last.
And if you want to be sure, set the module weight and/or implements hook_module_implements_alter() to ensure that your module is the one that is called last.
If you want to implement this in a theme then you only use the alter hook anyway and the module_implements_alter() won't work, but themes and especially the active themes are called last anyway.
The duplicate is possibly because the defaults include a suggestion for the field type and one for the field name, so if you name it the same as the field type, then you get the same suggestion twice