I usually take 6mg of melatonin but i bought some melatonin liquid that has 2.5 mg can i take 2doses of it to get some sleep?

Yes: 2 doses will make 5 mg, close enough to your previous dose. Sleep hygiene remains critical otherwise in allowing good quality sleep.

Same effect: Assuming the labeling is correct, these are roughly equivalent doses and should give you the same effect.

Dose high: When you exceed 5mg you get only risk of side effects, no benefit. Most effective dose is well below that.