No, the adverb “only” or “just” won’t be redundant if it is used to modify the verb “stayed” in that particular sentence. Instead, the sentence will now denote a value judgment on the part of the speaker or writer on the length of Monique’s stay in that place.
Specifically, by wanting to say “Monique has only stayed here for months” or “Monique has just stayed here for months,” the speaker considers Monique’s stay of a certain number of months as not that long. That’s not how those sentences would be constructed in actual practice, though. In English syntax, using the adverb “only” or “just” to modify a present perfect action reduces it to a simple past tense action—meaning that “only” or “just” makes the auxiliary verb “has” functionally unnecessary.
So, instead of saying “Monique has only stayed here for months,” the correct, idiomatic way of saying it is “Monique only stayed here for months.”
And instead of saying “Monique has just stayed here for months,” the correct, idiomatic way of saying it is “Monique just stayed here for months.”
What this means is that using the adverbs “only” and “just” does away with the present perfect tense altogether. In effect, when these adverbs are inserted in perfect tense constructions, it’s not them that become redundant but the auxiliary verb “has.”
