Now:

Crush tour:

I really don't think this qualifies as a duet tbh, Richie has always be very vocally present on this song (more than others) and if it really wasn't a duet earlier, why is it one now, it doesn't look like anything has changed, maybe he sings back up in 2 or 3 more lines.

They do sing at the same mic, don't know if that's why it's called a "duet".
The true duet of this song was the TLFR version they did during the HAND tour...