"Calendar to Show" option causes duplicate event to show up

I tried to make agenda to show an event using calendar. But the event is duplicated multiple times. I find out that it's because multiple calendar or account affects said duplication.

I tried changing the "Calendar to show" option and tried multiple setting, it works, the only problem was it has bug.

"Calendar to show" option seems to have bugged and automatically ticks options with same name multiple times as shown on screenshot 2.

Screenshot 1:

Screenshot 2:

Resulting in multiple ticked options (Screenshot 3)

Screenshot 3: