For yepnope. don't reexecute dupes.

once-filter.js

/**
 * Yepnope Once Filter
 *
 * by Alex Sexton
 * WTFPL
 *
 * Run this (ideally concatenated into the yepnope minified file) and any script that
 * you try to include twice will automatically be skipped (but your callback will still run).
 *
 */
(function ( yepnope ) {
var urlMap = {};

yepnope.addFilter(function ( resource ) {
  
  if ( map[ resource.url ] ) {
    resource.instead = function ( input, callback, chain, index, testresult ) {
      if ( testresult ) {
        // Determine callback, if any
        if ( callback ) {
          callback = ( {}.toString.call( callback ) == '[object Function]' ) ? callback : callback[ input ] || callback[ index ] || callback[ ( input.split( '/' ).pop().split( '?' )[ 0 ] ) ];
        }
        callback && callback( resource.origUrl, testResult, index );
        resource.autoCallback && resource.autoCallback( resource.origUrl, testResult, index );
      }
      else {
        resource.bypass = true;
      }
    };
  }
  else {
    map[ resource.url ] = true;
  }

  return resource;
});

})( yepnope );

It looks to me as if this will execute the callback immediately if the resource has already been queued, even if it hasn't already run. I was working on a similar feature but I thought the callback needed to be passed to chain.load instead of being executed directly by the instead function. Did I misunderstand when instead is called?

No you're absolutely correct. This was just a test, but that'd be a good thing to consider when I eventually make this a real addition. I appreciate the input.