eslint/lib/rules/no-redeclare.js

/**
 * @fileoverview Rule to flag when the same variable is declared more then once.
 * @author Ilya Volodin
 */

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = function(context) {

    "use strict";

    function findVariables() {
        var scope = context.getScope();

        scope.variables.forEach(function(variable) {
            if (variable.identifiers && variable.identifiers.length > 1) {
                variable.identifiers.sort(function(a, b) { return a.range[1] - b.range[1];});
                for (var i = 1, l = variable.identifiers.length; i < l; i++) {
                    context.report(variable.identifiers[i], "{{a}} is already defined", {a: variable.name});
                }
            }
        });
    }

    return {
        "Program": findVariables,
        "FunctionExpression": findVariables,
        "FunctionDeclaration": findVariables
    };
};