When you created the event did you tell it NOT to display on all days (underneath the end date field)?

I don't remember.
Can i edit the event now ?

I did edit it, to show only on one day in the calendar, but its not exactly what i want.
I want the event to display in both days in the calendar, but not 2 times in the module.
It is normal to say that an event occupies 2 days, but its not right to announce it 2 times.
I hope there is a solution to this.

There is a setting in the module parameters to override the value from the event - take a look at the options tabs