Refuse a path listed twice in a manifest (closes #170)

gen given arguments whose files share a path, such as gen a b with a.txt
in both, or gen . ., now fails while listing the files, before hashing
any, naming the path and both files. Builder.AddFile and
Builder.AddFileWithHash refuse a path already added. Loading refuses a
manifest that lists a path twice, compared byte for byte; fetch keeps
its own letter-case check. The Path Rules in docs/FORMAT.md say each
path appears at most once. The decode-size test listed one path 1000
times; each entry now has its own path of the same length.

Model: opus-5-5

mfer/deserialize.go

@@ -298,12 +298,21 @@ func (m *manifest) deserializeInner() error {
 	// extract path tomorrow — acts on a traversal or absolute path from an
 	// untrusted .mf. Reject loudly on the first offender rather than
 	// dropping entries, which would let a hostile manifest hide files from a
-	// check.
+	// check. A path listed twice is refused too: check would check the one
+	// file against both entries.
+	seen := make(map[string]bool, len(m.pbInner.GetFiles()))
+
 	for _, f := range m.pbInner.GetFiles() {
 		err = ValidatePath(f.GetPath())
 		if err != nil {
 			return fmt.Errorf("%w: %w", errInvalidManifestPath, err)
 		}
+
+		if seen[f.GetPath()] {
+			return fmt.Errorf("%w %q", errDuplicatePath, f.GetPath())
+		}
+
+		seen[f.GetPath()] = true
 	}
 
 	log.Infof("loaded manifest with %d files", len(m.pbInner.GetFiles()))