how to reduce the amplification of second order notch filter
The original poster must have an error in the measurements or the schematic does not represent what is built.

Here is a simulation of the OP's circuit. Notice that the gain is close to unity at frequencies significantly different from the notch frequency.



A likely problem is that the input voltage is not what the OP thinks it is. It is easy to mistakenly get twice the intended output from many function generators when feeding into a high impedance load. I have done it myself and seen others make the same error.

Most function generators have 50 ohms output impedance and assume that the output is loaded with 50 ohm.

Any display of voltage on the instrument will take that into account and allow for the fact that the voltage at the load will only 50% of the actual voltage generated internally and so display 1V for example when it would actually deliver 2V into a high impedance load.

Some generators have a setting for the load impedance when they will then take this into account and display the correct voltage. I have experience with Keysight generators that do this.

In this question I expect that the signal generator is actually giving out 2V because it has been set for 1V but is feeding a high-impedance, not 50 ohms.

Here is the schematic of the circuit I simulated: