While redstone current is ON, activating a button that is closer to the dispenser triggers it twice (on button Down and Up).

Repro Steps:
1. Place a dispenser and place some items in it.
2. Place 8 or so Redstone dust extending from it.
3. Place a switch at the end and activate it.
4. Place a button along the redstone line, nearer to dispenser.
5. Activate the button.

Result: The dispenser triggers once on button Down, and again on button Up.

Expected Result: Dispensers are only triggered when going from 0 power to 1 or more.