avoid getting multiple same path in findStaticAssets

when same paths is written many times in a html file, hash parameter is inserted many times to the path.

for example, when following code is in HTML file,
  &lt;img src="/img/common/t.gif"&gt;
  &lt;img src="/img/common/t.gif"&gt;
grunt-cache-bust genarate following html. 
  &lt;img src="/img/common/t.gif?cbcee5d614fdd537?cbcee5d614fdd537"&gt;
  &lt;img src="/img/common/t.gif?cbcee5d614fdd537?cbcee5d614fdd537"&gt;

So I fixed findStaticAssets method to avoid getting multiple same path.