One thing has nothing to do with the other.
If someone hits you it hurts even you've hit another person in the past.
Cheating is betrayal.
If someone has cheated and was led to "believe" they had been forgiven.
They might believe they are working to build a stronger relationship.
Sometimes the person doing the "forgiving" does so as a "reflex response" because they weren't able to handle the betrayal and a breakup all at once.
It's always best to take some time (week or more) before telling another you forgive them. After they told their mate they forgive them images haunt their minds, anger, and trust issues arise even though their mate may be bending over backwards to get back into their good graces.
"Revenge cheating" is petty stuff. Don't say you forgive someone unless you are prepared to start with a clean slate. Two wrongs don't make a right.