I saw the same problem as in this (Two accepted answers?) post on Math.SE. I want to pay attention on it, because it linked post is written that this bug has been yet fixed.
Quote comes from Two accepted answers? :
I noticed this anomaly after clicking the "load new answers" link on this question:
As far as I can tell, this was caused by the following sequence of events:
- I loaded the question with one accepted answer and left the tab open for a while.
- Meanwhile, a new answer was posted and the OP changed this to be the accepted answer.
- I returned to the tab, noticed the "load new answers" banner, and clicked it.
- The new answer got loaded, but now both were shown as accepted.
Of course, refreshing the page fixes the issue.