Duplicate elements in Queue for BFS Maze Algorithm
Yes.  There is no point having duplicates in the queue, so don't enqueue a position if (a) it has already been visited, or (b) it is already on the queue.

What happens if you skip checking whether it is already on the queue?  Depending on how the rest of your code works, this could slow down your implementation significantly (it could increase the asymptotic running time from linear to quadratic).