@raghuraj.muni, The first time you select the double tap, the Launcher gives you two options. One, an instant screen lock but fingerprint is not available.
Two, there is a few seconds delay, but you can use the fingerprint sensor. When you use this option, the home screen first blanks out, and tapping is disabled with only status bar visible (you can activate home screen during this time by tapping the capacitive buttons). After few seconds, the screen dims and locks. Now unlocking via fingerprint sensor is possible.