You have probably enabled mirror lockup.

The purpose of mirror lockup is to allow the slight vibrations caused by the movement of the mirror to dissipate before the shutter is opened. This is mostly an issue at shutter times between about 1/100 second and 1 second. Shorter exposures are over before the vibrations reach the parts of the camera that could cause motion blur from the mirror movement. Longer exposures are long enough that the vibrations last only a very short percentage of the total exposure time. Use of mirror lockup is normally accompanied by either the use of a wired cable release or by the use of a wireless remote control so that the camera does not need to be touched directly to release the shutter.

To disable mirror lockup, go into the 70D's menu setting under Shooting 2 (the second red tab with the little camera symbol). Scroll down to the bottom, select Mirror Lockup, change the setting from enable to disable, and press the Set button to apply the change.