Abbey Road Studios designed automatic double tracking and here’s a way to simulate that effect
First insert a delay across the vocal that doesn’t matter what the delay is in terms of short medium or long we’re going to set it to somewhere around 75 milliseconds it start it doesn’t really matter and if it has modulation we’re going to set the modulation some around pretty low and a rate of one Hertz between one and two and make sure that the mix is somewhere around 35 percent this is just where we’re going to start.
Next we’re going to do that is to automate a few parameters so we’re going to automate the delay. The depth of the modulation to automate, and the rate of the modulation. Vary your automation up and down as we are trying to simulate a tape machine’s wow and flutter. Don’t do very much, we just wanted very subtle changes.
You want to automate those three parameters delay and rate in order to get a more realistic doubling effect and by doing this way that you’re going to get it more realistic just about any other way.