Dumbbell Flye & Bench Press: Redundant?
Dumbbell flyes and dumbbell bench presses both involve horizontal shoulder adduction. They also create the most tension on the pecs when the forearm is parallel to the ground, which is 90-degrees to the force vector.
So they're basically the same exercise when it comes to how they load your pecs, thus making them redundant to one another. You don't need to do them in the same workout.
Dumbbell Bench Press
The reason you can use much more weight when doing a dumbbell bench press is because it allows the triceps to contribute. The weight is also much closer to your shoulder joints, which gives your pecs and front delts a much better mechanical advantage.
Do Cable Flyes Instead
Cable flyes aren't redundant to dumbbell bench presses because they load the pecs in a different manner than flyes with dumbbells.
Dumbbell flyes provide little to no force on your pecs when your wrists are directly above your shoulders. However, since cable flyes involve working against 45-degree force vector (the cables themselves), your pecs end up dealing with a great deal of load when your hands are directly in front of your shoulders.