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Hi,
Your documentation for the selection mode for ListBox shows two examples, one for mode Single and one for mode Multiple, both with showCheckbox set to true.
The one for mode Single appears to demonstrate a bug in that despite the mode being "Single" it allows multiple selection:

If showCheckbox is set to false then single/multiple correctly influence whether multiple can be selected so it appears to just be a bug manifesting when showCheckbox set to true.
I also noticed that the React version of the samples have case errors in the values for mode. Instead of the correct capitalized values of "Single" and "Multiple" they instead use incorrect lowercase versions "single" and "multiple" - see your documentation here (however even if correcting the case then, unsurprisingly as the same control code is used, the React version also has the same issue with allowing multiple selection for mode "Single").