redundant_shard_keys.js

db.getCollectionNames().forEach(function(n){
  var shardConfig = db.getSisterDB('config').collections.findOne({_id:"foursquare." + n},{key:1});
  if (shardConfig){
    var shardKeyNameParts = [];
    for (var keyPart in shardConfig.key) {
      shardKeyNameParts.push(keyPart + "_" + shardConfig.key[keyPart]);
    }
    var shardKeyName = shardKeyNameParts.join('_');
    var shardKeySizeMb = Math.round(db[n].stats().indexSizes[shardKeyName]/1024/1024);
    db[n].getIndexes().forEach(function(i){
      if (i.name.indexOf(shardKeyName) == 0 && i.name != shardKeyName) {
        print("Possible duplicates for " + n + ": " + tojson(i.key) + " with shardKey " + tojson(shardConfig.key) + " will save " + shardKeySizeMb + " mb");
      }
    })
  }
})