Race condition for multiple concurrent backups

[SchoolGuy]

Description

When recovering from #859 multiple backup pods are started simultaneously. This itself is not a problem. However, when finishing the backup an error is thrown.

Additional Context

I believe this is cosmetic as the restic repository appears to be intact. As such this should be safe to ignore?

Logs

2023-05-25T06:54:40Z ERROR k8up.restic.restic cannot sync snapshots to the cluster {"error": "snapshots.k8up.io \"c0ba34a1\" already exists"}
github.com/k8up-io/k8up/v2/restic/cli.(*Restic).sendSnapshotList
 /home/runner/work/k8up/k8up/restic/cli/backup.go:118
github.com/k8up-io/k8up/v2/restic/cli.(*Restic).Backup
 /home/runner/work/k8up/k8up/restic/cli/backup.go:46
github.com/k8up-io/k8up/v2/cmd/restic.doBackup
 /home/runner/work/k8up/k8up/cmd/restic/main.go:234
github.com/k8up-io/k8up/v2/cmd/restic.run
 /home/runner/work/k8up/k8up/cmd/restic/main.go:129
github.com/k8up-io/k8up/v2/cmd/restic.resticMain
 /home/runner/work/k8up/k8up/cmd/restic/main.go:113
github.com/urfave/cli/v2.(*Command).Run
 /home/runner/go/pkg/mod/github.com/urfave/cli/[email protected]/command.go:271
github.com/urfave/cli/v2.(*Command).Run
 /home/runner/go/pkg/mod/github.com/urfave/cli/[email protected]/command.go:264
github.com/urfave/cli/v2.(*App).RunContext
 /home/runner/go/pkg/mod/github.com/urfave/cli/[email protected]/app.go:333
github.com/urfave/cli/v2.(*App).Run
 /home/runner/go/pkg/mod/github.com/urfave/cli/[email protected]/app.go:310
main.main
 /home/runner/work/k8up/k8up/cmd/k8up/main.go:30
runtime.main
 /opt/hostedtoolcache/go/1.19.9/x64/src/runtime/proc.go:250

Expected Behavior

No stacktrace is thrown.

Steps To Reproduce

1. Have downtime for a schedule
2. Recover from the downtime
3. Observer that multiple backup jobs&pods are started
4. See the error in all pods that finish after the first one.

Version of K8up

2,7,1/4.2.2

Version of Kubernetes

v1.26.4+k3s1

Distribution of Kubernetes

k3s