"to defer the entire gain?"

You told us this: "The value of the rental property is close to the value of half of the duplex"

As you know, the entire purchase cannot be in the 1031. The like kind part of the exchange will be able to be computed, but we don't know what "close" means. Higher? Lower? Because it seems "close" to half the new property, which makes it "nearly" the entire rental half is either more or less. Depending.