xcodebuild "Multiple commands produce" in CI

Two build tasks claim to produce the same output file, and Xcode's build system refuses to guess which one wins. This commonly comes from a resource listed twice or a file present in both Copy Bundle Resources and Compile Sources.

What this error means

xcodebuild fails with "error: Multiple commands produce '.../App.app/config.json'" naming the duplicated output and the two tasks that produce it.

xcodebuild

error: Multiple commands produce
'/Users/runner/.../App.app/config.json':
1) Target 'App' has copy command from '.../config.json'
2) Target 'App' has copy command from '.../Resources/config.json'

Common causes

A resource added to the target twice

The same file is referenced in two places (two folder references, or a file plus a folder that contains it), so both copy it to the same output.

A file in both Compile Sources and Copy Bundle Resources

A source or asset listed in both phases produces the same output from two commands.

How to fix it

Remove the duplicate reference

Read the two producing commands the error names.
In the target's Build Phases, remove the file from one of the two phases or folder references.
Rebuild so only one command produces the output.

Avoid overlapping folder and file references

Do not add both a folder reference and individual files it contains; pick one so each output is produced once.

Build phase note

# In Build Phases > Copy Bundle Resources, ensure each
# resource appears exactly once across the target.

How to prevent it

Keep each resource referenced once across the target's phases.
Avoid mixing folder references with the individual files inside them.
Review project file diffs for duplicate resource entries before merging.