Category "bellman-ford"

In SPFA Shortest Path Faster Algorithm why does it have to check if current vertex is in queue before adding it to queue?

procedure Shortest-Path-Faster-Algorithm(G, s) 1 for each vertex v ≠ s in V(G) 2 d(v) := ∞ 3 d(s) := 0 4 push s into Q 5 w