It makes little sense to allow the creation of a PR with something that is already merged as there is not really content to review anymore. It is also a bit inconsistent since PRs are automatically closed when a manual merge is done. Letting the possibility to create a PR that should be directly marked as merged is not really useful.
Issue has been spotted by @gbonnefille in https://tuleap.net/plugins/tracker/?aid=15082#followup_68102