As a general rule, do not mention a specific time that the action ocurred. "Already", as you mentioned, is fine; "at six o'clock" is too specific. You would say, however, "I have had my breakfast at 6:30 every morning ever since I can remember", the governing time reference being "since I can remember".
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Vctory Ong(1)I have already had my lunch at six o'clock.
As Philip has already explained, as a rule, present perfect do not require "time stamps".
(2)I have had my lunch at six o'clock.
(3)I have had my lunch, so I feel very full now.
(4)I have had my lunch at six o'clock,so I feel very full now.
(5)I had my lunch at