Sometimes Gautama Buddha's the 10th Avatar of Vishnu, sometimes he isn't. It really depends on the telling. Also, him becoming an Avatar I thought was more Hindus trying to reincorporate Buddhism into the fold and dissolve it as an independent tradition. It's what we do.
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It's not really a crossover if Buddhism basically originated as an offshoot movement and lifted Hindu cosmology wholesale, only removing references to the Allmighty.