To clarify: App Store Connect used to reject uploads if they had the same version/build number combo as a build that was previously uploaded. Now it no longer rejects these, but silently increments the build number so that the submission succeeds. But what I want is the old behavior of rejecting the upload if a build is submitted with the same build number

For anyone else coming here: Using xcodebuild: Add the key manageAppVersionAndBuildNumber as a bool with a value of 0 to your export options plist file (provided to xcodebuild with the -exportOptionsPlist argument. Using Xcode GUI: Uncheck the box that says 'Manage Version and Build Number'