Caching multiple copies of outputs for each source file

I'm using Buck2 for large C++ builds. For various reasons, I can't use RBE. It appears that if I build a source file on commit A, then again on commit B (where the contents of the source file or its headers have changed), then build it again on commit A, there's no cached artifact for it. It appears that the output from commit B has clobbered the output from commit A from the first build. It would be great if Buck2 could keep multiple copies of outputs for each source file (or maybe I'm just missing a flag that Buck2 already has).