Hello @biozshock, thank you for opening this issue with a reproducer.

May I ask what is the use case that requires to not map repeated inner fields?

@HeahDude I have reused one of application types that has defaults set as not mapped.

I wouldn't say that this is the bug in the symfony itself, but it would be helpful to have an exception or other error of some sort, just to save time for the developer in case they use not mapped repeated fields.
Maybe some validation in configureOptions is the right place, but I'm not sure if that one can catch that inner fields have default not mapped or not.

We agree this is not a bug, as this is the expected behavior and inner fields should always be mapped. But adding some kind of validation to it can be complex.

So to resume IIUC, your case is something like:

$builder
    ->add('email', RepeatedType::class, [
        'type' => CustomEmailType::class,
    ])
;

where CustomEmailType has a default mapped=false option, so you need to do:

$builder
    ->add('email', RepeatedType::class, [
        'type' => CustomEmailType::class,
        'first_options' => ['mapped' => true],
        'second_options' => ['mapped' => true],
    ])
;

in order to have it working properly, is that right?

Yes, that's how i solved this 15 minutes ago. Came here to write the workaround.

But i would say that setting mapped=true for both fields as default values you can not override would be a perfect solution.