Someone else's contacts downloaded to my phone

UPDATE to my first message below: I spent about an hour in a chat with a great AT&T rep this morning. He'd never heard of such a problem before, but he got to the source. Somehow my Google account had synced someone else's contacts with my account. I went into my Google account on computer, found and deleted all those mystery contacts, they also disappeared from my phone. (My real contacts weren't affected.) Now, did it answer the question why this happened? NO. But in searching it out, I discovered that when I synced my gmail account with my phone, Google also synced a lot of other features (which i don't use) without my being aware of it. I spent time cleaning up a lot of stuff, including changing passwords, and can only hope it doesn't happen again. This was my weird experience for the week.

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I turned on my phone (Samsung Galaxy s7 Edge) the other day and pulled up contacts only to discover about 35 or 40 contacts that are NOT mine. Mine are still there, but I now have someone else's, too. My impulse was to go through and delete them, but before that I want to find out what is going on? And have my contacts also been sent to someone else's phone? No, I haven't called any of the new random numbers; I'm hoping ATT or Samsung or someone more tech savvy than I can explain this first.

My purse was stolen a couple of months ago, so this is a replacement phone provided by the insurance company. It did not come in Samsung packaging (I was told Samsung wouldn't allow it???) but was assured that it wasn't a used or refurbished phone. I'm not at all sure about that. There are subtle differences in how this one works, but most of all, what's with this contacts mess? Does anyone have any information?