TYPO3 Core

Tested this in a FlexForm select field with 7.6.9

<settings.test>
                        <TCEforms>
                            <label>LLL:EXT:test/Resources/Private/Language/locallang_be.xlf:flexForm.settings.test</label>
                            <config>
                                <type>select</type>
                                <renderType>selectMultipleSideBySide</renderType>
                                <foreign_table>tx_test_domain_model_test</foreign_table>
                                <size>1</size>
                                <maxitems>9999</maxitems>
                                <minitems>1</minitems>
                                <multiple>1</multiple>
                                <autoSizeMax>30</autoSizeMax>
                                <size>10</size>
                                <show_thumbs>1</show_thumbs>
                                <wizards>
                                    <suggest>
                                        <type>suggest</type>
                                    </suggest>
                                </wizards>
                            </config>
                        </TCEforms>
                    </settings.test>

I think your problem is another.

At first i thought i have the same bug .. you have used 'multiple'.

The 'multiple' option does not say you can select multiple elements, it says you can select any element multiple times.

So, remove the 'multiple' option it and it will work :)

Oops, you are right, of course :)
I guess we can therefore close this.