bug: emoji name collision is allowed when importing emoji

Created by: Amelia

💡 Summary

if importing an emoji from a remote instance, the name isn't checked for collisions, so two emojis are able to share the same name.

🥰 Expected Behavior

this behaviour should be prevented, with an error, or an option to rename the imported emoji

🤬 Actual Behavior

emoji is allowed to be imported, causing a name collision

📝 Steps to Reproduce

import an emoji with the same name as an existing emoji

💻 Frontend Environment

* Model and OS of the device(s): Fedora Kinoite
* Browser: Firefox Flatpak
* Server URL: ambrosia.moe
* Misskey: 13.x.x

🛰 Backend Environment (for server admin)

* Installation Method or Hosting Service: baremetal
* Misskey: commit 42bf8e5
* Node: 20.4.0
* PostgreSQL: docker
* Redis: docker
* OS and Architecture: Debian 11 x86_64

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