This happens when you have multiple devices from which you answer phone calls (one or more desk phones, smartphones, tablets, desktops, or laptops). When you login to the Dialpad app on from a computer, smartphone, or tablet, that devices will be "registered" with your account and by default, all devices will ring when you receive a call. Even if you answer the call on one device, the devices will register the call as missed.
If you want multiple devices to ring simultaneously when you get a call, then there is no way to avoid this. However, you can easily configure any device to not ring when you receive new calls. To do that, login at https://dialpad.com. Once you are at the Your Profile page, scroll down and click Your Devices and uncheck the box in the Ring This Device column or click the three dots on the far right to delete the device.