This is a babel plugin that makes duplicate keys in object literals like the following example be transpiled into valid ES5 code. This works around a bug in Babel: https://phabricator.babeljs.io/T2462
var x = a: 5 a: 6;
Without this, object literals with duplicate keys will be incorrectly transpiled by Babel into non-ES5 compatible code that will fail to parse in older browsers (including Safari <=8 and Chrome <=41).
Duplicate keys are often introduced into code by mistake, but they are allowed by the ES2015 standard. This plugin allows all browsers to consistently parse the transpiled code. If you want to avoid introducing duplicate keys into your code, then use a linting tool.
This plugin is included in the babel-preset-es2015-dupkeyfix preset, so just switch to that instead of using "babel-preset-es2015".