It depends on the room you're in. If there are lots of hard surfaces, you're going to get a lot of reverb. You can mitigate that by putting up some curtains or draping blankets over surfaces that aren't in the frame. If it's a true echo effect, as in you can actually talk over yourself, then it sounds like you might need to consult a troubleshooting guide on the mic and camera as it's entirely possible that the on-board mic is still picking up sound and there's a slight delay between it and the external cardioid mic. This also sometimes produces what's called a chorus effect, which is similar to reverb but has a denser sound.