TYPO3 Core

In TCA is is possible to configure a field that the same objekt can be referenced more than once. This settings is called "multiple" and is documented here:
http://docs.typo3.org/typo3cms/TCAReference/Reference/Columns/Select/Index.html#multiple

Extbase does not support this feature (yet).

TCA configuration:

'products' => array(
    'exclude' => 1,
    'label' => 'LLL:EXT:myext/Resources/Private/Language/locallang_db.xlf:tx_myext_domain_model_product.products',
    'config' => array(
        'type' => 'select',
        'foreign_table' => 'tx_myext_domain_model_product',
        'MM' => 'tx_myext_order_product_mm',
        'size' => 10,
        'autoSizeMax' => 30,
        'maxitems' => 9999,
        'multiple' => 1
    )
)

In BE it is possible to put the same item in the list for example three times.

In FE there will be only one item in the list.