In many equation editors (e.g. MS Word 2007 and Daum Equation Editor), there's a double square bracket. Is there a TeX equivalent for
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If you only want then occasionally, perhaps you can just roll-your-own by using a little bit of negative space, like this
If you'll be using them a lot, try using a package like
Note especially the fancier display for double parentheses.
As a side note, DetTeXify is getting better and better at recognizing these symbols, so you can check there for more package-based options.