There are two identical phone icons with two identical names on the list of devices found in Find My iPhone. One device "moms phone" is not on, the other is. Where did this device come from? Does my Wife have a second phone?

We have about 9 iDevices, and recently in the Find my iPhone app I realized there were two identical devices listed, each with the name "Moms phone." However, one of them was located, the other was listed as offline. My wife switched phones about 2 years ago, but that second posted phone hasn't been there that long. Is there a way I can find out what the seriial numbers are for a given device listed in Find My iPone? Otherwise, I'll have to wonder if my wife is secretly keeping a second phone, or that somebody has backed up their phone with her account info.