What happens when another person/company buys the same character as I did from the asset store and uses it commercially to their game, Can I sell a game using the same character?

Thousands of people buy the same characters from the asset store, how can all of them use the same character in their game?
Wont it lead to copyright issues?

No, it won’t. Keep in mind that you never really buy the asset. What you buy is a usage license. The assetstore license grants you a non-exclusive eternal usage right. The important part is non-exlusive. That means the copyright holder (the creator of the asset) can sell as many licenses as he wants to as many customers as he wants. You don’t have to worry if you have a valid license. The asset creator would get in trouble if he in addition is selling exclusive licenses. Such licenses are generally much more expensive but it guarantees the license holder that he has exclusive access to that asset.

Please keep in mind that we are not lawyers and can’t give you any legal advice. However I would just recommend you read the license agreement that you have accepted when you bought something on the assetstore.