Mocha constructor creates duplicate entries in mocha.globals

[pascalpp]

Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

• When creating a mocha instance with options.globals, duplicate globals are created in mocha.options.globals.
• When adding globals on an existing mocha instance, more duplicates are created.

Steps to Reproduce

const Mocha = require('./lib/mocha')

// create a mocha instance with options.globals
const mocha = new Mocha({ globals: ['foo', 'bar'] })

// duplicate entries are created
console.log(mocha.options.globals)
// [ 'foo', 'bar', 'foo', 'bar' ]

// call mocha.globals with the same global entries
mocha.globals(['foo', 'bar'])

// more duplicate entries are created
console.log(mocha.options.globals)
// [ 'foo', 'bar', 'foo', 'bar', 'foo', 'bar' ]

Expected behavior: [What you expect to happen]

mocha.options.globals should only contain unique entries.

Actual behavior: [What actually happens]

mocha.options.globals contains duplicate entries.

Reproduces how often: [What percentage of the time does it reproduce?]

100%

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 6.1.4
• The output of node --version: 10.15.3
• Your operating system
  • name and version: Mac OS X Mojave
  • architecture (32 or 64-bit): 64bit
• Your shell (e.g., bash, zsh, PowerShell, cmd): bash
• Your browser and version (if running browser tests): n/a
• Any third-party Mocha-related modules (and their versions): n/a
• Any code transpiler (e.g., TypeScript, CoffeeScript, Babel) being used (and its version): n/a

Additional Information

c.f. discussion on #3914

[pascalpp]

Will attempt to resolve this in #3914