My galaxy s5 keeps heating up?

Ok. My galaxy s5 is always heating up when I'm on it. This is a replacement phone for my other galaxy s5. The reason why I got a replacement was because my other galaxy s5 was heating up alot. But now my replacement phone is heating up also. I don't think it's the phone I thinks it tha battery. I called at&t and took the phone to them and they said nothing was wrong with it. And then the next day I called at&t back cuz the phone was heating up and when I mean by heat up i mean it was burning hot! So I called them and they sent me the replacement phone and I put the battery and the backcover into my replacement phone and as soon as I download everything and put everything on to that phone (the replacement phone) it was already getting hot. I'm so fed up with this! Please help.

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