A Duet on TikTok is an interactive way to repost a video, as it allows you to post any video from a TikTok creator side-by-side with your video. To use this feature, click on the "Share" icon of the video you'd like to duet and then click on the "Duet" button from the pop-up menu if the creator has enabled the feature. Record yourself side-by-side with the video and then post the Duet, which will include the original creator's username in the caption as attribution.