It’s not, on Leetcode. I submitted it as an answer and it worked. In the case where the number is repeated more than once (m times), you have an array of n+m elements. Divide the sum by m, subtract, problem solved.
The actual sum is 8, and the 4th triangular number is 10, so the duplicated number according to your algorithm must be -2 (or some fraction thereof, depending on how you calculate m).