assets/js/vendor/backbone-relational.js
function( model, id ) {
			var coll = this.getCollection( model ),
				duplicate = coll && coll.get( id );

			if ( duplicate && model !== duplicate ) {
				if ( Backbone.Relational.showWarnings && typeof console !== 'undefined' ) {
					console.warn( 'Duplicate id! Old RelationalModel=%o, new RelationalModel=%o', duplicate, model );
				}

				throw new Error( "Cannot instantiate more than one Backbone.RelationalModel with the same id per type!" );
			}
		}
function( model, id ) {
			var coll = this.getCollection( model ),
				duplicate = coll && coll.get( id );

			if ( duplicate && model !== duplicate ) {
				if ( Backbone.Relational.showWarnings && typeof console !== 'undefined' ) {
					console.warn( 'Duplicate id! Old RelationalModel=%o, new RelationalModel=%o', duplicate, model );
				}

				throw new Error( "Cannot instantiate more than one Backbone.RelationalModel with the same id per type!" );
			}
		}
Check if the given model may use the given `id`
@param model
@param [id]