diff --git a/snapshot/gc/gc.go b/snapshot/gc/gc.go
index 433cbc998..66560c581 100644
--- a/snapshot/gc/gc.go
+++ b/snapshot/gc/gc.go
@@ -8,6 +8,7 @@
 
 	"github.com/pkg/errors"
 
+	"github.com/kopia/kopia/fs"
 	"github.com/kopia/kopia/internal/kopialogging"
 	"github.com/kopia/kopia/internal/units"
 	"github.com/kopia/kopia/repo"
@@ -20,9 +21,11 @@
 
 var log = kopialogging.Logger("kopia/snapshot/gc")
 
-func findInUseContentIDs(ctx context.Context, rep *repo.Repository, used *sync.Map) error {
-	w := snapshotfs.NewTreeWalker()
+func oidOf(entry fs.Entry) object.ID {
+	return entry.(object.HasObjectID).ObjectID()
+}
 
+func findInUseContentIDs(ctx context.Context, rep *repo.Repository, used *sync.Map) error {
 	ids, err := snapshot.ListSnapshotManifests(ctx, rep, nil)
 	if err != nil {
 		return errors.Wrap(err, "unable to list snapshot manifest IDs")
@@ -33,6 +36,8 @@ func findInUseContentIDs(ctx context.Context, rep *repo.Repository, used *sync.M
 		return errors.Wrap(err, "unable to load manifest IDs")
 	}
 
+	w := snapshotfs.NewTreeWalker()
+	w.EntryID = func(e fs.Entry) interface{} { return oidOf(e) }
 	for _, m := range manifests {
 		root, err := snapshotfs.SnapshotRoot(rep, m)
 		if err != nil {
diff --git a/snapshot/snapshotfs/snapshot_tree_walker.go b/snapshot/snapshotfs/snapshot_tree_walker.go
index 38518eaa4..473ae2725 100644
--- a/snapshot/snapshotfs/snapshot_tree_walker.go
+++ b/snapshot/snapshotfs/snapshot_tree_walker.go
@@ -12,10 +12,15 @@
 	"github.com/kopia/kopia/repo/object"
 )
 
+// TreeWalker holds information for concurrently walking down FS trees specified
+// by their roots
 type TreeWalker struct {
 	Parallelism    int
 	RootEntries    []fs.Entry
 	ObjectCallback func(oid object.ID) error
+	// EntryID extracts or generates an id from an fs.Entry.
+	// It can be used to eliminate duplicate entries when in a FS
+	EntryID func(entry fs.Entry) interface{}
 
 	enqueued sync.Map
 	queue    *parallelwork.Queue
@@ -26,8 +31,8 @@ func oidOf(entry fs.Entry) object.ID {
 }
 
 func (w *TreeWalker) enqueueEntry(ctx context.Context, entry fs.Entry) {
-	oid := oidOf(entry)
-	if _, existing := w.enqueued.LoadOrStore(oid, w); existing {
+	eid := w.EntryID(entry)
+	if _, existing := w.enqueued.LoadOrStore(eid, w); existing {
 		return
 	}
 
@@ -54,6 +59,7 @@ func (w *TreeWalker) processEntry(ctx context.Context, entry fs.Entry) error {
 	return nil
 }
 
+// Run walks the given tree roots
 func (w *TreeWalker) Run(ctx context.Context) error {
 	for _, root := range w.RootEntries {
 		w.enqueueEntry(ctx, root)