I was able to eliminate the hundreds of duplicate definition warnings by changing the Intent Class Generation Language build setting (INTENTS_CODEGEN_LANGUAGE) from Swift to Objective-C. I also had to import the auto generated intent headers into my Swift bridging header, since most of my intent usage is in Swift.
I suspect this is only an issue in projects that still use Obj-C, since all the errors only originated in Obj-C files that imported my project's Swift header, and as @CannedMango pointed out, the duplicate definitions seem to be due the new async/await functions translating to the same Obj-C method signatures as the earlier completion-based functions.