iZotope has introduced Vocal Doubler, a freeware vocal doubling effect in VST, VST3, AU, and AAX plugin formats for compatible digital audio workstations on PC and Mac.
Vocal Doubler sports a modern-looking user interface with a simple control scheme. While the user interface isn’t resizable, it is large enough for comfortable use on high-resolution screens. After testing Vocal Doubler, my impression was that moderate amounts of stereo separation and modulation worked the best for doubling lead vocals. The doubling effect sounds convincing and natural with just the right amount of pitch drifting when the modulation amount is set to around 50%.
Read more about this here: https://bedroomproducersblog.com/2018/10/08/izotope-vocal-doubler/