[Feature Request] Warning before duplicate items are imported

[northword]

Describe the feature

当前行为是条目已经被保存到库中后才提醒重复的，后续操作为合并重复项。

理想的行为是准备保存到库中时，就提醒，选择是否保存。

The current behavior is to be prompted for duplicates after the item has already been saved to the library, with the subsequent action being to merge the duplicates.

The ideal behavior is to be prompted when you are ready to save to the library, choosing whether to save or not.

Additional context

• https://github.com/corajr/zotero-prevent-duplicates/blob/master/preventduplicates/chrome/content/preventduplicates/preventDuplicates.js
• https://forums.zotero.org/discussion/78188/prevent-duplicate-during-import
• Show modified save icon if URL/DOI exists in library zotero/zotero#1007