This may clarify the difference:
In this use, the word "just" is time-related. It expresses recentness, as in "I have just arrived -- 5 minutes ago!" So, "I have just come to my house," tells me that you have been at your house only a short amount of time.
"I have *already* come to my house," does not have that same kind of temporal meaning. It expresses only that you completed the activity of coming to the house, and now you are at the house.
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