Let $m$ be the minimum element of the original array. Every current element is at least $m$, since merging two values at least $m$ produces a value at least $m+1$.

The last operation therefore gives at least $m+1$. To obtain exactly $m+1$, one of the two elements immediately before that operation must equal $m$.

No merge can create $m$, so this element must be an original minimum that has never been merged.