lib/ignore: Ignore duplicate lines in .stignore

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4350
LGTM: AudriusButkevicius, calmh

lib/ignore/ignore_test.go

@@ -872,6 +872,7 @@ func TestLines(t *testing.T) {
 
 	!/a
 	/*
+	!/a
 	`
 
 	pats := New(fs.NewFilesystem(fs.FilesystemTypeBasic, "."), WithCache(true))
@@ -886,6 +887,7 @@ func TestLines(t *testing.T) {
 		"",
 		"!/a",
 		"/*",
+		"!/a",
 		"",
 	}
 
@@ -898,5 +900,33 @@ func TestLines(t *testing.T) {
 			t.Fatalf("Lines()[%d] == %s, expected %s", i, lines[i], expectedLines[i])
 		}
 	}
-
+}
+
+func TestDuplicateLines(t *testing.T) {
+	stignore := `
+	!/a
+	/*
+	!/a
+	`
+	stignoreFiltered := `
+	!/a
+	/*
+	`
+
+	pats := New(fs.NewFilesystem(fs.FilesystemTypeBasic, "testdata"), WithCache(true))
+
+	err := pats.Parse(bytes.NewBufferString(stignore), ".stignore")
+	if err != nil {
+		t.Fatal(err)
+	}
+	patsLen := len(pats.patterns)
+
+	err = pats.Parse(bytes.NewBufferString(stignoreFiltered), ".stignore")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if patsLen != len(pats.patterns) {
+		t.Fatalf("Parsed patterns differ when manually removing duplicate lines")
+	}
 }