Shaanxi

From a modified form of the Hanyu Pinyin romanization of Chinese 陝西 /陕西 (Shǎnxī, “West of the Shan [Pass]”). The double-a spelling, used certainly to avoid homography with Shanxi (山西, of shān rather than shǎn), is not a feature of Hanyu Pinyin and cannot be observed outside reference to Shaanxi (see also: Ningshaan, Shaanbei, Shaan-Gan-Ning, Shaanzhong). It is likely inherited from the pre-Pinyin Latinxua Sin Wenz system devised and employed by Communist linguists, which was toneless and employed "irregular spellings" for undesirable homographs. The pairs Shaansi (陝西 /陕西) and Shansi (山西) appear (for the first time?) in the influential Sin Wenz primer 《中國話寫法拉丁化——理論·原則·方案》 (1935). An alternative theory is that the double-a spelling is from the Gwoyeu Romatzyh romanization system, where the third tone is spelled by doubling a vowel (Shaanshi 陝西 /陕西 vs. Shanshi 山西), but this is less likely considering the history of Gwoyeu Romatzyh and Sin Wenz, including the political and ideological rivalry between the two systems.

来源：wiktionary