Function replication_filesystem_mount_collision 

pub fn replication_filesystem_mount_collision(
    source: &Backend,
    dest: &Backend,
) -> Option<String>

Two DISTINCT filesystem repositories that share the same in-pod backend.path cannot ride one replication mover pod: the Job mounts each repo’s volume at its own path, and two volumeMounts at one mountPath make the pod spec invalid — a failure that would otherwise surface only as a reconcile-time Job-create error. [backend_target_key] deliberately keys filesystem targets by (path, volume) so this pair PASSES the self-target check (different volumes = genuinely different repos); the mount collision is a separate, mover-topology constraint, so it gets its own guard.

Returns the shared path when both backends are filesystem-backed and their paths are equal (regardless of volume); None otherwise. Callers deny at admission and defensively re-check at Job-spawn time.

use kopiur_api::backend::Backend;
use kopiur_api::validate::replication_filesystem_mount_collision;
let fs = |path: &str, pvc: &str| -> Backend {
    serde_json::from_value(serde_json::json!({
        "filesystem": { "path": path, "volume": { "pvc": { "name": pvc } } }
    }))
    .unwrap()
};
let a = fs("/repo", "a");
assert_eq!(
    replication_filesystem_mount_collision(&a, &fs("/repo", "b")),
    Some("/repo".to_string())
);
assert_eq!(replication_filesystem_mount_collision(&a, &fs("/repo-dst", "b")), None);