iPhone serial number

Hey, after i bought my iphone 13 7months ago and it suddenly have a motherboard issue. Luckily its still in warranty so i got to the nearest apple authorised service centre. After a week i got my iphone back and they said that they had repaired the motherboard. But then i notice there are 2 iphone 13 under my apple id with different serial number. So may i know if this is my old phone or they actually gave me a new phone?