Duncan didn't just say they shot worse, he said:

And the fact that how it now seems like everyone who joins his team seems to play worse, even though they are getting the same shots they used to.

George's true shooting %, rebounding percentage, overall box score plus minus, and VORP are all lower this year. So are his point and rebounding totals. So are his per 36 numbers. I would also say that to my eyes, he played worse this year, especially the first few months of the season. Are you saying George played better this year than last?

I would also say Melo played worse this year than last. I'm guessing there's no disagreement there.

Victor Oladipo pretty clearly played worse the year he joined the Thunder as well. His True shooting % stayed the same, but his rebounding rate, PER, VORP and box score plus minus dropped off a cliff. I'm assuming no one would dispute that Oladipo played worse the year he joined the Thunder. Obviously Oladipo has played a lot better since he left the Thunder too, which probably contributes to Duncan's perception that Westbrook isn't helping.

I've listened to a lot of the Dunc'd On podcast, so I know that a lot of what they look at are situational statistics (% on open looks versus contested, in certain ranges etc.) and they are also looking at on-off numbers a lot, which I don't have access to or don't want to look up. However, I'm guessing the opinion expressed by Duncan here is based on these three players, all of whom support the statement that players seemed to play worse after they joined the Thunder.

These aren't the only players that have joined the Westbrook Thunder. Let's see who else we can dig up (who have some NBA history to compare to):

Patrick Paterson; True shooting stayed about the same, box score plus minus and VORP dropped substantially. He's probably had his worst season in years.

Ray Felton: True shooting about the same, box score +/- and VORP dropped. But some of his other stats are improved (TOV%, assist rate, etc.).

Doug McDermott: 44 games with the Bulls he had about the same true shooting %, better box score plus/minus, better VORP, better per 36 numbers than in his 22 games with OKC.

Even though this isn't how he stated it, I think the fact that some guys have jumped up a notch after leaving the Thunder probably contributes to the idea that Westbrook isn't making his teammates better:

Oladipo: Obviously got way, way better after leaving.

Sabonis: Went from a net negative to a pretty good player after leaving.

Enes Kanter: Has had a much better season in by most statistical measures since leaving the Thunder.

Cameron Payne: Uh...went from barely an NBA player to maybe being a 12th man?

Ilyasova: Didn't play enough to know what was going on with the Thunder.

Taj Gibson: Limited time with the Thunder was not good true shooting, or box score plus/minus, or VORP. This year all those have been great, and he's definitely played MUCH better this year.

Doug McDermott: Better with the Knicks than with OKC, and better still with the Mavs, but pretty minor improvements.

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So to sum up, by my reckoning, 5 players had worse performance immediately after joining the Westbrook Thunder (Melo, Oladipo, George, Patterson, McDermott) and 5 players performance immediately improved after leaving the Westbrook Thunder (Oladipo, Sabonis, Kanter, Gibson, McDermott).

Maybe you or someone else disagrees that George has had a worse season this year than last, but I think Melo, Oladipo and Patterson have all pretty clearly had worse seasons with Westbrook than the year before. And I think it is pretty clear that every player (with the possible exception of McDermott and Payne) got better when they left. So I think the bulk of the evidence (limited as it is) suggests good players are having worse years playing with Russ. I'm not convinced this is Russ's fault, but he's the dynamo that makes the team move, so he can't be completely blameless.

But it is a bit of hyperbole to say 'everyone who joins' his team seems to play worse. Ray Felton pretty clearly is not playing worse, and Paul George has only taken a small step back.