Enemies push player a ridiculous amount.

I (Finally) finished my enemy character, but whenever i have more than one, the duplicates are acting weird. they behave fine, but when a player touches a duplicate, it sends the player away, really fast. it’s like if you play minecraft and you have a huge amount of entities in one block, and you walk into them, the same will happen (or it did, last time i played) if you need to see my project, i can give it to you.
Here is a video of the but, if you can, also tell me why the enemies can walk into each other if other objects can’t:
Dropbox link to Video: https://dl.dropboxusercontent.com/u/27099275/Enemy%20Push%20Bug.mov
If you want, i can upload a different video that might make it more understandable…?

I fixed it by setting the mass to 0