What causes a Unity coroutine running twice?

StartCoroutine is called more than once without stopping the previous run, so multiple instances of the coroutine run concurrently, doubling its effects.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix a Unity Coroutine Running Twice

Quick answer: Stop the existing coroutine before starting a new one, guard against starting while one is running, and track the coroutine handle.

A coroutine running twice is overlapping starts. Stopping the old one fixes it. Here is how.

How to fix it

1. Stop before starting

Before StartCoroutine, stop any existing instance (StopCoroutine on the stored handle) so you do not have two running at once. Starting again without stopping the previous run duplicates its behavior.

2. Guard against re-entry

Track whether the coroutine is already running with a flag and skip starting it again while it is, so a repeated trigger does not launch a second concurrent instance.

3. Store the coroutine handle

Keep the handle returned by StartCoroutine so you can stop exactly that coroutine. Without the handle you cannot stop the specific run, making it hard to prevent duplicates reliably.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

A crash you can name from its stack trace is a crash you can usually fix in minutes.