When I consider a question, the answer is straight up and honest based on information from the question. This one presents a dilemma of sorts! The markings are what should be there for 18k gold. The only thing suspect there is the double marks, including both 18k and 750. This is not a critical point but most Italian jewelry is only marked with one or the other mark go here.
Sure, some exceptions occur and when the bracelet was made “might” be a reason for both marks. European jewelry was marked with numerical stamps (i.e. 750) before those marks became accepted standards with consumers in the USA. Who knows the reason for both marks? Generally, the maker’s mark is (by law) next to the quality mark. Is there a maker’s mark or some indication besides “Italy” for a manufacturer? If not, then the bracelet is truly suspect.
The marks are right but the acid test showed green, the typical result of a gold-filled or plated article with a base metal like brass underneath. The green comes from reaction with acid and the copper element in the underlying metals. Gold items of 10k to 18k and higher simply do not show green. This is part of the dilemma. The double safeties also are a small problem, since costume jewelry generally does not have good quality clasps or double safeties.
Now, we have to question if the acid test was done correctly and on what part of the bracelet. It is possible that the bracelet is not gold and does have gold clasp parts, if attached with loops (jump rings) and not connected by soldering directly to the bracelet.
Where is the quality stamps located? Are the markings on the back only on the clasps parts or located somewhere else? An acid test done correctly will involve not only the clasp parts but some other area of the jewelry item. The item is rubbed on a slate stone hard enough to dig slightly into the jewelry and place both surface gold and deeper metal on the stone. Or, the acid is applied directly to the piece. Either way, the result will not be green for gold items.
Without the bracelet in hand, I simply cannot tell if it is really 18k gold. If not, then apparently enough trouble was taken by the manufacturer to make it look authentic and be sold falsely as 18k when it is not.