Fix deduplicate for non-printable (and non-ASCII) characters.

tests/t1910-deduplicate.sh

@@ -6,7 +6,7 @@ Ensure we can deduplicate items successfully.
 '
 . ./test-lib.sh
 
-cat >> todo.txt<<EOF
+cat > todo.txt <<EOF
 duplicated
 two
 x done
@@ -35,4 +35,24 @@ test_todo_session 'deduplicate without duplicates' <<EOF
 TODO: No duplicate tasks found
 EOF
 
+cat > todo.txt <<EOF
+normal task
+a bold action
+something else
+a bold action
+something more
+EOF
+test_todo_session 'deduplicate with non-printable duplicates' <<EOF
+>>> todo.sh deduplicate
+TODO: 1 duplicate task(s) removed
+
+>>> todo.sh -p ls
+2 a bold action
+1 normal task
+3 something else
+4 something more
+--
+TODO: 4 of 4 tasks shown
+EOF
+
 test_done