Reasons for a voltage follower introducing voltage gain?
There are a few problems. The first is that you have not read the data sheet (or if you have, you did not understand/follow it). The maximum power supply voltage range is 16 volts total. So your +/- 12 volt supplies very likely damaged the chip. No more than +/- 8 volts is allowed.

Now, let's say you've got a new chip and you're running on +/- 5 volts. Where (exactly) are you connecting - input voltage and the - output voltage. I suspect that you are just letting them float, and that will not do. You must connect your input voltage reference to the power supply common, and measure your output voltage with respect to that common as well. 







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