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The 'double-voice' effect is present in both performances separately - this makes the live-MR track in the performance more convincing.
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About 'live MR':
A live MR track contains a pre-recorded voice track that is normally different from one used in the studio version of a song. Live MR voice tracks are mastered in a way that it sounds different from the CD recording, and that it won't be filtered when making MR-removed audios so live MR vocals remain, thus making it seem as if the artists are singing live.
The term was put into the spotlight when the set list for July 12, 2015 edition of Inkigayo was leaked (
http://2.bp.blogspot.com/-XI49beBTYeU...), which showed Girls' Generation was to use 'live MR' on that show, whereas other groups were simply using 'MR'. The latter is the norm: an instrumental track with background vocals, which the go-to option when an artist performs live (Note: There's also 'AR', an instrumental without any type of vocals. EXID used it by accident once:
https://www.youtube.com/watch?v=ComCR...). However, 'live MR' is meant to lip-synced on. A quick review of the performances of that day should show that all groups who were noted as 'MR' did sing live, whereas Girls' Generation did not.
While still lip-syncing, live MR is a different case: besides being different from the CD version, it's also easily confused as live singing, especially by MR removed videos, thus showing live vocals when there really is none.
These comparisons are made just to show how and when live MR works. There is no intention to discredit or bash any of the artists depicted in these videos.