SHA256, which Dropbox uses, has around 10^77 possible hashes. That's 100 trillion quadrillion quadrillion quadrillion quadrillion possible values. So I wouldn't worry about hash collisions if I were you. If a hash collision happened easily in SHA256, that would be very big news for the security community, and much more serious services than Dropbox would be affected.
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SHA256, which Dropbox uses, has around 10^77 possible hashes. That's 100 trillion quadrillion quadrillion quadrillion quadrillion possible values. So I wouldn't worry about hash collisions if I were you. If a hash collision happened easily in SHA256, that would be very big news for the security community, and much more serious services than Dropbox would be affected.