A popular trick in many deeper forms of contemporary dance music, such as post-dubstep and deep house, is to layer vocals with pitchshifted versions of themselves to create weird, almost androgynous-sounding harmonies.
While some producers employ dedicated vocal processing tools to do this, it's actually extremely quick and easy to create this effect yourself, using nothing more than your DAW's ability to pitchshift audio.
The key is selecting the correct algorithm: use the wrong one and the processed version of the vocal won't sit comfortably with the original, and the effect will be ruined. We're using Ableton Live for this demonstration, so if you're following along with a different DAW, you might need to consult its documentation to find out how to access its various pitchshifting algorithms.
Step 1: Open Live, set the project tempo to 128bpm and drag your vocal onto an audio track. Duplicate the audio track by right-clicking the track name and selecting Duplicate. Name the first track Main Vocal, and the second track Processed Vocal.
Step 2: Double-click the clip on the Processed Vocal track to bring up its Clip view. Change the Warp Mode from Beat to Complex Pro. Solo the track and set the Transpose in the Clip View down to -12. The vocal is pitched down an octave.
Step 3: Complex Pro mode adjusts the audio's formants to compensate for the change in pitch, but it still sounds a little unnatural. Turn the Formants parameter down to 0.00, and unsolo the track. Now balance the level of the Main Vocal and Processed Vocal tracks to taste.