By the way, what do you mean saying "they are not duplicate"? Are the original papers behind these Zotero items different papers? If they links to the same original paper but were recognized as items of different types you should select that of incorrect type (to select just an item of interest you might try to select/deselect while pressing Ctrl button) and change the type to make items consistent in type.
If the papers are of different origin, you might go by changing titles and DOI for Zotero to recognize them as separate items.

https://forums.zotero.org/discussion/109637/zotero-mistakenly-identifies-the-identical-doctoral-thesis-and-journal-article-as-duplicate-entries

我也遇到了相同的困惑。以下是我的一些认知：
1. 我感觉这个问题仅出现中文里面，外文没有。原因是，人家识别DOI。并且，学位论文和期刊论文应该都有DOI；
2. 你看了上面的回答，人家是Zotero 目前使用标题、DOI 和 ISBN 字段来确定重复项。如果这些字段匹配（或缺失）考虑年份。
如果实在是强迫症，不行了自己修改DOI(没有了，自己造一个)。你回发现，可以了。

I also encountered the same confusion. Here are my observations:

1. I feel this issue only arises in Chinese contexts, not in foreign languages. The reason is that foreign systems recognize DOIs, and both dissertations and journal articles should have DOIs.

2. As mentioned in the previous response, Zotero currently uses title, DOI, and ISBN fields to identify duplicates. If these fields match (or are missing), it considers the publication year.

If you’re truly obsessive about this, you can modify the DOI—even create a fake one if there isn’t an official one. You’ll find it works.