Example of custom merge strategy for integrating fetched data into an enyo.Collection that may have records that already exist in the shared store (with another enyo.Collection) as per the discussion: http://forums.enyojs.com/discussion/1830/how-to-merge-records-from-different-collections.

SampleCollection.js

enyo.kind({
    name: "sample.Collection",
    kind: "enyo.Collection",
    // this may not be preferred but for example we overload fetch to always default
    // to the new _reuseOrAdd_ strategy unless specified
    fetch: function (opts) {
        opts = opts || {};
        opts.strategy = opts.strategy || "reuseOrAdd";
        // ensure we're passing our new arguments
        this.inherited(arguments, [opts]);
    },
    // we use a custom merge strategy to check if the record exists in the store
    // and reuse it if it does
    reuseOrAdd: function (records) {
        // we need to be able to figure out what the primary key is that will be
        // used to check uniqueness in the store, since you may already know this
        // you could hardcode it in
        var store = this.store,
            proto = this.model.prototype,
            pk    = proto.primaryKey;
        // we still have to check to make sure this is in an array form
        // so it is usable directly with a single record as well
        if (records) {
            records = enyo.isArray(records)? records: [records];
            for (var i=0, data, record; (data=records[i]); ++i) {
                // we use the _findLocal_ method of the store to see if a record
                // with these keys already exists
                record = store.findLocal(this.model, data);
                if (record) {
                    // if it does we replace the entry with the existing one
                    records[i] = record;
                    // optionally you could update the value if that was something you wanted
                    // to do as well such as...
                    record.setObject(data);
                }
            }
            // now that we know we either have new records or existing records
            // and we want to be able to safely call this whether adding or merging
            // with existing data we let the default merge now do its thing
            this.merge(records);
        }
    }
});