TYPO3 Core

I made a TCA config to allow items mutiple times via the option 'multiple' => 1.

The config looks like this:

'type' => 'select',
'renderType' => 'selectMultipleSideBySide',
'foreign_table' => 'tx_testmultiple_domain_model_condition',
'MM' => 'tx_testmultiple_test_condition_mm',
'MM_hasUidField' => true,
'size' => 10,
'autoSizeMax' => 30,
'maxitems' => 9999,
'multiple' => 1,

In the backend I can add items multiple times but when I'm trying to get all items in the frontend all duplicate items are removed.
Is this a bug or do I need additional configurations?

I made a litte extension for testing:
http://hosting8098.af933.netcup.net/test/test_multiple_1.0.0_201712271223.zip
Same problem exists in TYPO3 7.6 also.