Please help me! (Beginner)

Hi, i am a 14 year old trying to figure out blender. I am working on my first model which is a shark man. But i have this issue that when i extrude, it ends up duplicating vertecies, so that there are 2-4 vertecies in the same spot. And this causes my model to be connected very strangely and unusable. Please help me prevent this in the future.
Thank you for all help:D

In edit mode on the tool shelf press “Remove doubles”

if they are layered on top of each other it should remove all but one- or remove the manually.

Remove doubles can also be reached using Specials menu (w), so you can just select all in edit mode and press w then r to do it quickly. It is however important to keep track of everything you click and not to extrude things unnecessarily - that is the only way to avoid this in the future.

Would suggest watching these

they all contain useful information which is correct, uses actual terminology, and also talks about the issue you’re asking about

For future questions, use images to help explain the issue and include an example .blend file that contains the issue so others can find out what causes it, and use the file to explain it back to you ask clearly as you asked.