Why your Mac won't open a second copy
When you open an app that is already running, macOS brings the existing window to the front instead of starting a new copy. Most chat and AI apps on the Mac, including Slack, Claude, and Discord, are built on Electron, which adds a second lock: the app keeps one profile folder with your login in it, and refuses to start twice on the same folder.
So even if you force macOS to launch a second copy, it usually just focuses the first window, still signed in to the same account.
The free workaround: Terminal
For Electron apps you can start a second copy by hand and point it at a separate profile folder. For example, for Slack:
open -n -a "Slack" --args --user-data-dir="$HOME/Library/Application Support/Slack-2"
This works for many apps, with some catches:
- You have to run the command every time you want the second copy.
- Both copies have the same name and icon in the Dock, so it's hard to tell them apart.
- Sign-in links that come back from your browser (for example "Sign in with Google") go to whichever copy macOS picks, often the wrong one, so logging in can fail.
- Both copies share notification and permission settings, because macOS sees them as the same app.
The one-click way: Duplex
Duplex is a small Mac app that does this properly. It makes a lightweight clone of the app with its own identity, so macOS treats it as a separate app.
- Install Duplex with Homebrew or the notarized download below.
- Click "New Instance…" and pick the app from your Applications folder.
- Name it, for example "Slack Work", and choose an icon. A colored badge makes it easy to tell apart in the Dock.
- Launch it and sign in to your second account. It opens from the Dock or Applications like any other app from then on.
Each copy keeps its own login, cookies, and settings. The original app stays untouched, keeps updating itself, and your instances follow its updates automatically.
Which apps work
Duplex works with Electron apps, which covers most desktop chat, AI, and productivity apps on the Mac, such as:
Duplex checks the app when you pick it and tells you if it isn't supported. Native Mac apps such as Safari, Messages, or WhatsApp can't be duplicated this way, and Mac App Store versions of apps aren't supported, so download the app from its developer's website instead.
Get Duplex
Your first instance is free, forever. A $9 one-time license unlocks unlimited instances on up to 2 Macs. Requires macOS 13 or later.
brew tap bnimit/tap brew trust bnimit/tap brew install --cask duplex
Questions
Can I run two instances of any Mac app?
Only Electron apps, which include Slack, Claude, Codex, Discord, Signal, Notion, Obsidian, and VS Code. Native apps such as Safari or Messages can't be run twice with separate logins. Duplex tells you when you pick an app it can't duplicate.
Why does open -n not give me a second account?
open -n starts a second process, but Electron apps lock their profile folder, so the second copy either quits or shares the same login. You also need --user-data-dir pointing at a separate folder, and even then sign-in links may go to the wrong copy.
Does a second copy take up double the disk space?
No. Duplex makes a copy-on-write clone, which shares its disk blocks with the original. Finder shows the full app size, but a Claude instance costs about 6 MB of real disk.
Does Duplex change the original app?
No. The original is never modified and keeps updating itself. Each instance follows its updates automatically the next time you launch it.
Is Duplex free?
Your first instance is free, forever, which is all you need for a second account. A $9 one-time license unlocks unlimited instances on up to 2 Macs. There is no subscription.
Is it safe?
Duplex is native Swift, notarized by Apple, and collects no analytics. The source is public on GitHub, so you can read exactly what it does.