What gets copied
The entire instance folder, including mods, configs, worlds, screenshots, options.txt, resource packs, shader packs, and the per-instance GDLauncher metadata. The duplicate is independent of the original; changes on one don't affect the other.
When to use Duplicate
- Testing mod changes safely. Want to try adding a new mod but worried it'll break things? Duplicate first, test on the copy, only update the original once you know it's stable.
- Branching a playthrough. Reached a turning point in a world and want to try going both ways? Duplicate, then play each branch separately.
- Trying a different mod loader on the same base. Duplicate, switch the duplicate's loader (see How to Switch Mod Loaders).
- Cheap backup before risky changes. Updating a modpack version mid-playthrough is one example; duplicate first, do the update on the copy.
What doesn't get copied
Things outside the instance folder are not duplicated. Your accounts and launcher-level settings stay the same (they're not per-instance anyway). Some modpacks have content under the app data path or elsewhere; those aren't duplicated either, but it's rare enough not to matter.
Locked vs unlocked, paired vs unpaired
Duplicating preserves the lock state and the modpack pairing. A locked modpack instance produces a locked modpack duplicate, still pointing at the same modpack. Both can be updated independently via Change Modpack Version on the instance's Settings tab.
If you want the duplicate to no longer track the source pack, open its Settings tab and click Unpair in the Modpack Info section after duplicating.
Disk space caveat
Duplicates are full filesystem copies, not symlinks or copy-on-write. Each duplicate doubles the disk usage of the original. For 50 GB modpacks, this can add up quickly; consider what you actually need to keep around long-term.