Which one is correct i already have had my lunch or i had my lunch ??

Both are correct. Just need to see in what context you are using it.

Case 1.

“Just Had my dinner”    (Uses: Recent past.)

I have just had my dinner. (You recently finished the said task)

Case 2.

“Had my dinner” (omitted JUST” (Uses: not time bound/ specific.)

I have had my dinner.

Case 3.

“Just have my dinner” (Uses: 1 for future reference)

2) To stick to that very task.

Case 4.

“Have my dinner” (omitted “JUST”)

Uses: present indefinite tense/ simple present tense ( habitual / regular activity/frequency)

E.g.: I have my dinner everyday/ often/regularly/without a skip/ late at night/ very light