Written for Synergy 3 (v3.8.4), checked 20 October 2026.
Click Yes. Synergy 3 stops the other program and starts sharing your keyboard and mouse. Only one keyboard and mouse sharing app can run on a computer at a time, because they all use the same network port.
Which message did you see?
- Already running: another keyboard and mouse sharing app, such as Barrier or Input Leap, is running. To stop this coming back, quit that app or remove it from your startup apps.
- Legacy core running: an old version of Synergy 1 is still running. To stop this coming back, quit it or uninstall it.
- Core already running: another copy of Synergy 3's own sharing process is running, for example left over after a crash, or Synergy 1 is running on this computer. Click Yes to replace it. If it's Synergy 1, quit it or uninstall it to stop this coming back.
Click Cancel to leave the other program running; Synergy 3 can't share your keyboard and mouse until it stops.
If the message keeps coming back after you click Yes, restart the computer. If that doesn't help, send us a diagnostic report: Getting diagnostic info and logs
Already running while copying Synergy 3 to Applications (macOS)
- Click OK.
- Click the Synergy 3 icon in the menu bar.
- Choose Exit.
- Open the new copy of Synergy 3 again, and click Accept when it offers to copy itself to the Applications folder.
This message says Another Synergy 3 is already running. Please exit Synergy 3 first. It shows when you open Synergy 3 from outside the Applications folder, for example from the downloaded disk image, while another copy is already open.
Using Synergy 1? Its Synergy is already running message means Synergy 1 itself is already open: Messages and what to do