repeated selection in multiple selection

[themajix]

1. while the dropdown is open in a multiple select control, hit the tab button
2. the first choice in the list will be added to the selection list
3. open dropdown again and click the tab button
4. the first choice will be added to the selection list again

Repeating steps 3 and 4 will cause the first choice to be added to the selection list indefinitely.

Expected behavior: Each choice can be added to the selection list for a maximum of one instance only.