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I am confused by the question: there are a lot of synonyms (words with approximately the same denotation) in almost every language. They often have different histories: and the same is the case here. avani originally came from a root meaning to drive, and ila from a root meaning to flow, and via meanings like course and stream bed came to mean ground in general.
No idea! But in English, one synonym for Earth could be Soil or Ground, and another could be World, so I don't find it surprising.