What causes a Unity duplicate-key exception?

You called Add with a key that already exists in the dictionary, which throws, because Add does not allow duplicates and the code did not check or expected unique keys that were not unique.

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 “An item with the same key has already been added” in Unity

Quick answer: Check ContainsKey before Add, use the indexer to add-or-update, or use TryAdd, and ensure the keys you generate are actually unique.

A duplicate-key exception is adding an existing key. Checking first fixes it. Here is how.

How to fix it

1. Check before adding

Use ContainsKey before Add, or use TryAdd which returns false instead of throwing on a duplicate. Add throws if the key exists, so guard it when keys might repeat.

2. Use the indexer to upsert

If you want to add or overwrite, assign with the indexer (dict[key] = value) instead of Add. The indexer updates an existing key rather than throwing, which is often the intended behavior.

3. Ensure keys are unique

If keys are supposed to be unique but are not, the real bug is the key generation. Check why a duplicate key arises — a shared id, a collision — and fix the source so keys are genuinely unique.

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.

The errors you never hear about are the ones quietly costing you players. Visibility turns them into a worklist.