I have had this happen several times with another medication.
I've been on it now for... um... 20? years, and at various times different pharmacies in different places have given me 4 or 5 different brand names. However the dosage of the actual drug has always been according to the prescription, and I have never noticed any difference in the action of the drug.
Since Metformin may (in some people) produce some unpleasant side effects, I would take it carefully, and check with your doc/pharmacy if you have any problems develop. Other than that, just carry on as normal.