The Hammers managed to find the back of the net through Cheikhou Kouyate in their meeting at Upton Park in December, but Arsenal were able to see out a 2-1 victory.
"Facing Arsenal is more difficult at their own place," Allardyce told reporters. "On the day at our ground there was not much between the two teams.
"They have a few weaknesses but if we can expose them and take our chances. It is down to how well we perform on the day. They have adapted to change the way they play to win football matches. We must take our chances and defend strongly."