It is a bug
Well, it is clear that it is a bug. Your calculation is right: it should be +6 damage. You can ask about bugs on D&D Beyond here.
In the first page, there is a similar issue in this thread.
One staff member has responded:
Unfortunately there is currently a way to select the fighting style option multiple times, though we plan to resolve it.
To resolve it, please go edit the fighting style and move it from ARCHERY to CHOOSE AN OPTION and then repeat until it stays on CHOOSE AN OPTION.
Then select the desired fighting style as normal.
So, it seems it is a problem with Fighting Styles and, when you select it, you may, by accident, select it more than once, so, instead of one instance of “Dueling”, it seems you got 4 instances, stacking a bonus of +8.