Sometimes people talk about making a lead vocal sound thicker by “doubling” it – copying the track and applying some subtle effect to the copy (such as delay or pitch shift), and then sometimes panning the two tracks opposite each other.
Personally I never use such doubling tricks – if I want to emphasise the vocals in a particular section I’ll either add backing vocals (usually harmony, but occasionally unison), or copy the vocal to a new track and add a longer delay – to add depth rather than density.
Personally, most doubling tricks just sound like gimmicks to me. Sometimes if I really want a unison double and the singer isn’t around anymore I’ll use some of the other takes (with editing/correction if necessary). However, if you really want a unison double AND your singer isn’t around anymore AND you only got one take (ie, you can’t use the additional takes as doubles – which I recommend doing if you can) then your options are limited. Whatever you do it’ll still sound like a single take, a single performance.