I personally believe that everyone deserves a second chance. Everyone makes mistakes once in a while. As long as the person who did you wrong is trying to change themselves and what they did wrong then give them a second chance. Actions always speak louder than words.
I do believe that there are some times where people do not deserve a second chance. If someone cheats on you, lies to you or walks out of your life with no reason at all then you should not give them a second chance unless they prove themselves worthy enough for one.
I, myself, make a lot of mistakes in friendships like not talking to my friends every day, not replying for weeks on end, not telling them when something is bothering me that they do. I tend to allow people back in my life time and time again which is allowing that person to hurt me time and time again. That is why second chances can be bad. Giving a person a second chance is allowing that person to keep hurting you and keep doing what they were always doing.
This being said, second chances can sometimes be a good thing. Second chances are good when people changed their actions and want a second chance to show you that their actions have changed. I have only given second chances to about five people my whole life because I tend to make friends and date people who I can see a future with and get along with forever.
The mistake that people make often is giving second chances to the wrong people. If someone hurts you in any way and then tries to come back into your life, do not let them. Some people will say that they have changed and they will even act changed for a period of time, but eventually they will go back to their old ways. Unfortunately, there really is no telling if a person is actually changed or not.
Second chances are good and bad. Sometimes people make mistakes and are willing to change in order to be in your life. There are people who are not willing to change, but still want a spot in your life. Try your best not to be tricked into thinking people are really different when they are not unless they actually put in effort to change.
The bottom line is that if people want to be a part of your life, they will accept your wishes and change what they did wrong.