also haveก็ adds emphasis or a sense of “also,” while มี means “to have” or “there is/are.” Depending on context, it can mean “do have” or “there are.”
TH: ไม่ต้องซื้อใหม่หรอก เราก็มีอยู่แล้ว
EN: There’s no need to buy a new one; we already have one.