Sorry, but I can't reproduce this in a simple test. I get:山田太郎1965a; 山田太郎1965bYou'll have to provide more detail. If you are doing this with a custom style, try switching to another style that uses author-date disambiguation and see if it happens there. Also please post the code of the style to http://gist.github.com. If you want us to test the style locally, please tell us exactly what is in the fields (Title, Author, etc) of the two entries that are producing the fault.

(Edit: Ah, one more thing: are you using Zotero 2.0.9 or Zotero 2.1.1 for this?)

Thanks for responding so soon. I'm using zotero 2.1.1 for firefox 4 in linux. The style I'm using is "American Meteorological Society (dev)", which was installed from the "Get additional style" link within zotero. At first I thought this error only occurs when adding Chinese citations, but now I find it happening even for some of mine English citations. Here I have 3 groups of citations, listed as follows:
Group 1:
@article{_lagfdwam_1992,
title = {LAGFD-WAM 海浪数值模式: I: 基本物理模型},
volume = {14},
number = {5},
journal = {海洋学报},
author = {袁业立 and 潘增弟},
year = {1992},
pages = {1--7}
}
@article{_lagfdwam_1992,
title = {LAGFD-WAM 海浪数值模式: II. 区域性特征线嵌入格式及其应用.},
volume = {14},
number = {6},
journal = {海洋学报},
author = {袁业立 and 华锋},
year = {1992},
pages = {12--24}
}
Group 2:
@article{wornom_mpi_2002,
title = {An MPI quasi time-accurate approach for nearshore wave prediction using the swan code. II. Applications to wave hindcasts.},
volume = {44},
issn = {0578-5634},
number = {3},
journal = {Coastal Engineering Journal},
author = {S. F Wornom and R. Allard and Y. L Hsu},
year = {2002},
pages = {257-280}
}
@article{wornom_mpi_2002,
title = {An MPI quasi time-accurate approach for nearshore wave prediction using the swan code. I. Method.},
volume = {44},
issn = {0578-5634},
number = {3},
journal = {Coastal Engineering Journal},
author = {S. F Wornom and {D.J. S} Welsh and K. W Bedford},
year = {2002},
pages = {247--256}
}
Group 3:
@article{moon_effect_2004,
title = {Effect of Surface Waves on {Air-Sea} Momentum Exchange. Part I: Effect of Mature and Growing Seas},
volume = {61},
journal = {J. Atmos. Sci.},
author = {{Il-Ju} Moon and Tetsu Hara and Isaac Ginis},
year = {2004},
pages = {2321--2333}
}
@article{moon_effect_2004,
title = {Effect of surface waves on air-sea momentum exchange. Part II: Behavior of drag coefficient under tropical cyclones},
volume = {61},
number = {19},
journal = {Journal of the Atmospheric Sciences},
author = {{IL-JU} Moon and I. Ginis and T. Hara},
year = {2004},
pages = {2334--2348}
}
@article{moon_effect_2004,
title = {Effect of surface waves on Charnock coefficient under tropical cyclones},
volume = {31},
journal = {Geophys. Res. Lett.},
author = {{I-J} Moon and I. Ginis and T. Hara},
year = {2004},
pages = {L20302}
}
Groups 1 and 2 are both wrongly interpreted by zotero when adding into LibreOffice. They produce "(袁业立 1992aundefinedb)" and "(Wornom et al. 2002aundefinedb)", respectively. On the other hand, the 3rd group doesn't have this problem, it produce "(Moon et al. 2004a; Moon et al. 2004a; Moon et al. 2004c)" correctly.
I also tried using "Chicago Manual of Style (Author-date format)". This time all things goes fine, even Chinese citations are correctly processed.
Thus I guess it's probably the style's problem. As "American Meteorological Society" is a style contained in the "Zotero Style Repository", would you please make it more stable? In fact, this style is widely used in oceanography-meteorology field, and is among the very few styles this field currently have in the zotero style repository.
Additionally, please add more styles in the ocean-atmosphere discipline, which is a very large and active field. Famous journals in this field include Journal of Physical Oceanography (published by the American Meteorology Society), Journal of Geophysical Researches, Journal of Fluid Mechanism, and many more.
Here I would like to express my gratitude to you. Zotero is an excellent research tool, and I enjoy it so much. I'm happy to see you maintain this software so actively. Hope that zotero will become perfect in a short time.
Thank you!

No problem; thanks for the test data, this will be very useful in getting to the bottom of the problem. We'll certainly get this fixed: more soon.

Separately, as you are handling Chinese citations, I can't resist a plug for the multilingual version of Zotero. It's a labor-of-love experiment, meant only for testing at this stage. That said, the code base relatively stable, and slouching toward eventual merger into Zotero main; any feedback from people with multilingual requirements would be very welcome at this stage.

I'll post here again when the American Meteorological Society style is working correctly with your test data.

Thank you. Expecting your good news. I hope I'll be able to try zotero multilingual in the following days and give you some feedback.