Default value with two sets of parentheses instead of one

((0)) is how it will appear within the catalog views or if you generate a script in SSMS. So I think when you see that it's most likely a generated script and not hand-written that way from scratch.

That said, there is nothing different between (0), ((0)) or even (((((((((((((0))))))))))))) ... assuming I counted right, extra parentheses don't change the meaning, just readability.