@contrex,
Quote:They are not both correct. ... The word "just" always comes after the word "have".
They are both correct / They both are correct.
The normal neutral placement is between 'have' and the lexical verb but there is some freedom for placing 'just' elsewhere.
Quote:You can insert other words such as recently, never, probably, always, etc to modify the phrase, but they always come after the "have"
I have never seen a penguin.
I have probably failed my examinations.
I have always told the truth
I have recently bought a car.
I have usually received the goods the day after I ordered them.
"I just have received the goods" is non standard. Is it the sort of error that non-native-speaking learners make.
If you wish to convey the idea of a shortfall you might say
"I asked for ten bottles but just got eight".
Again, the above represent the normal neutral placement. But,
I never have seen a penguin.
I have seen a penguin never.
Never have I seen a penguin.
Probably, I have failed my examinations.
I probably have failed my examinations.
I have failed my examinations, probably.
I always have told the truth.
Always, I have told the truth.
I have told the truth, always.
I recently have bought a car.
Recently, I have bought a car.
I have bought a car recently.