Can we remove duplicates faster than we can sort?
It is a classical result that the element distinctness problem requires $\Omega(n\log n)$ comparisons in the comparison model (the one used to analyze sorting algorithms); in fact, it also requires $\Omega(n\log n)$ time in stronger models such as algebraic decision trees, in which we are allowed to compute the sign of arbitrary bounded degree polynomials (rather than just $x_i - x_j$).

The element distinctness problem asks whether all elements in the input are distinct. This is clearly easier than your problem, since you can just compare the sizes of the input and output arrays. Hence in these computation models, you cannot do asymptotically better than $n\log n$.