Minecraft Redstone only let every second signal through?

I have built an elevator where the cabins always go back to the lower floor. The presence of the elevators is indicated by items in donors, i. Only if one of the donors is an item can it be sent again. When the elevator leaves, the item is shot by an upstream mechanism in the "on the way" dispenser. However, the observer I put down on the elevator shaft gives not only an output when the elevator comes back (and thus shoots the item into the "present" dispenser), but also when the elevator starts, which he already does the departure is reported as arrived. Is there a simple mechanism that lets only every second output of the donor through?

With a Sticky Piston and another block that can direct the signal.

Guide the signal from the observer to a Sticky Piston with a Redstone Block on it.

The Redstone block is then only "extended" every other time → it only gives a signal every second time.