What is double tracking vocals? It is making your vocalist sing the exact same part of a song on a second track. It is then blended with the main vocal track to elevate its sound. However, this process is both underrated and overrated in the music recording world.
Sad to say, there are some who overdo it to the extent of making the track sound unnatural. Hence, it is constantly misused. A lot of people are not aware of the effect of doubling in their songs and how much variety available when mixing.
Read more about this here: https://sundownsessionsstudio.com/double-tracking-vocals/