I think American English would accept "I already posted it", but in British English it would be "I have already posted it".

I already posted

Even though both are correct, the british would use 'I have already posted'

Hi there. Both sentences appear to be correct. However, “I have already posted” is commonly used. I’m available to answer more of your questions feel free to message me.

Actually, There is no difference. and I think American English would accept "I already posted it", but in British English it would be "I have already posted it". Ex : - After a few hours, I have already posted more than I have in the past few months. or - I already post information about political parties, the election commission and other things about politics.