Good point. Rung higher.
By signing Diaw to this contract, the Spurs now face a hard cap at $74 million. And yeah, right now the max contract Duncan can get from the Spurs is something like $16.5 million. Once the Spurs use the rest of the MLE and the BAE (which I still think they'll most likely have to use to fill out the roster), Duncan won't be able to re-sign for more than $14 million.
But I'm sure he's already agreed to a number south of that.