The GBC ChaosEgaddulatorRadulatorFinderLocatorrator, better known as the Emerald Radar Radar, or simply Radar Radar is a tracking device that Professor E. Gadd created. It shows the location of the original Emerald Radar, and is first seen in Episode 5. It was lost in the battle during Episode 6, and Wario and Waluigi picked it up. They have been using it to follow the Heroes ever since.
This item is as redundant as expected, considering that a second radar could just as easily locate the owners of the original. This is pointed out by Sonic and even E. Gadd himself agrees. However, it's probably for the best, considering who picks it up. It was quite possibly a choice of Alvin-Earthworm's, as he once stated that Wario and Waluigi were simply 'Comic Relief', or at least would be until the next saga, or until he could give them a better role.