Longest non-repeating subsequence

Design an effective algorithm that for a given sequence of integers x₁, x₂, .... , x_n finds a longest continuous subsequence with no repeated entries.

That is values a,b maximizing b-a, satisfying

 1) 1⩽a<b⩽n

 2) x_c ≠ x_d for every a⩽c<d⩽b.