I can confirm this is still happening for me too in Xcode 13.1 RC.
The duplication seems to be with the SWIFT_AVAILABILTY function. My Siri extensions have a deployment target of 12.3, but in the duplication method I can see the availability setting to iOS 15.
- (void)handleCreateDailyTask:(OP1CreateDailyTaskIntent * _Nonnull)intent completion:(void (^ _Nonnull)(OP1CreateDailyTaskIntentResponse * _Nonnull))completion;
- (void)handleCreateDailyTask:(OP1CreateDailyTaskIntent * _Nonnull)intent completion:(void (^ _Nonnull)(OP1CreateDailyTaskIntentResponse * _Nonnull))completionHandler SWIFT_AVAILABILITY(watchos,introduced=8.0) SWIFT_AVAILABILITY(macos,introduced=12.0) SWIFT_AVAILABILITY(ios,introduced=15.0);