For instance, if you got a hit by people in the street accidentally, you might say "算了算了“. It means you think it doesn't matter or you don't care about this offence. "忘了" means "forget", basically we don't repeat "忘了忘了”to express you forget sth. However, it can show that you forget sth with impatience. eg."Did you remember the homework?""Of course not, I'm not the good boy(忘了忘了，我又不是乖乖仔)"

"算了算了/suànle suànle" and "忘了忘了/wàngle wàngle" is totally different in chinese. for example "算了算了/suànle suànle" .most of the time it is mean something is not what you expected,and you can't change the result,you will say "算了算了" ,There's some sense of disappointment "忘了忘了/wàngle wàngle", i don't catch you maybe you want to say "完了 完了/wanle wanle"

算了 表示 不计较了 忘了 表示 不记得了

算了，means let it pass 忘了means I forget it

算了，means let it pass 忘了means forget it