Skip identical entries when listing chunks

The prune command can remove redundant chunks (chunks with the same chunk id
but at different subdirectory level).  However, if the same chunk appears
mutliple times in the listing returned by the storage, it will be treated as
a redundant chunk and thus removed.

src/duplicacy_snapshotmanager.go

@@ -2427,7 +2427,13 @@ func (manager *SnapshotManager) pruneSnapshotsExhaustive(referencedFossils map[s
 	}
 
 	allFiles, _ := manager.ListAllFiles(manager.storage, chunkDir)
+	uniqueFiles := make(map[string]bool)
 	for _, file := range allFiles {
+		if _, found := uniqueFiles[file]; found {
+			continue
+		}
+		uniqueFiles[file] = true
+
 		if len(file) == 0 || file[len(file)-1] == '/' {
 			continue
 		}