I attacked Zombies for multiple times with a pipe-wrench. After killing them (fell down, lifelessand the cursor chaged to the loot icon), the corpse suddenly dissapears and suddenly an identicly looking zombie runs towards me from about 20 or 30 meters away to attack me again (might be the very same).
After killing the 2nd Z, this one stays dead, while the corpse dissapears within 10 seconds or so.