Checks for duplicate field names in a field selection list.

Normalizes field names using the input formatter before checking for duplicates. Throws {:duplicate_field, field_name, path} if duplicates are found.

Validates that a field exists in the given field specs.

Throws {:unknown_field, field_name, error_type, path} if not found.

Validates that nested fields are non-empty for fields that require selection.

Throws appropriate errors if validation fails.