If you are holding the clutch in and revving it from a stop and letting go, that is not double clutching. That is slipping the clutch. And if you rev it up high and let it out slowly you are going to kill your clutch in a few hundred miles. Even in 1st or any gear, you don't want to rev it up and let it out slowly. When you hear of people slipping the clutch, its more of a 3500rpm, and let it out slower than just letting go of it, but you still let it go out faster than you would just cruising around.
Double clutching is when you start like normal and then when you shift gears you clutch in, shift to neutral, clutch out and blip the throttle, clutch in and shift to the next gear. Its meant for when cars did not have synchros and you had to manually get them synchronized. NOT needed on any modern manual transmission unless the synchros are ****.