If I see him again I will slap him. What will you do Harry?
1-I already did it.
2-I already did that.
I think '2' means:
a-I saw him and I slapped him.
I think '1' could have that meaning but it could also mean:
b-I have already seen him and done whatever it was I had to do.
Am I correct?
I agree with you. "I already it" doesn't tell us what he did, simply that he has done it. "I already did that" means "I already slapped him".
Note that in BrE, they would probably say "I have already done it" and "I have already done that".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.