Double download of remote cache in some edge cases

[bpaquet]

How to reproduce the issue?

Buildkit version: moby/buildkit:buildx-stable-1

Dockerfile

FROM debian:bookworm-slim
RUN apt-get update

Start a local registry

docker run --network=host --rm -p 5000:5000 registry:2 2>&1 | grep -v 'level='

First build (please adapt the IP of the registry :) )

docker buildx rm --builder=foo; docker buildx create --name=foo
docker buildx build --builder=foo . --cache-to type=registry,mode=max,ref=10.133.91.224:5000/foobar,registry.insecure=true --load

Second build, which load from the registry and upload it into another image

docker buildx rm --builder=foo; docker buildx create --name=foo
docker buildx build --builder=foo . --cache-from type=registry,mode=max,ref=10.133.91.224:5000/foobar,registry.insecure=true --load --cache-to type=registry,mode=max,ref=10.133.91.224:5000/foobar2,registry.insecure=true

You can check the log of the registry, we can see some layer are downloaded twice during the second build. I was expecting only one.