sort: Fix inconsistent sort orderg under i18n-collator with equal sorting keys.

src/uu/sort/src/sort.rs

@@ -2655,7 +2655,13 @@ fn compare_by<'a>(
     if global_settings.precomputed.fast_locale_collation {
         let a_key = a_line_data.collation_key(a.index);
         let b_key = b_line_data.collation_key(b.index);
-        let cmp = a_key.cmp(b_key);
+        let mut cmp = a_key.cmp(b_key);
+        // If collation keys are equal, fall back to lexicographic comparison
+        // This can be the case for inputs like `01` and `0_1`, which have equal keys
+        if cmp == Ordering::Equal {
+            // Reversing the order to match sort's sorting behaviour
+            cmp = b.line.cmp(a.line);
+        }
         return if global_settings.reverse {
             cmp.reverse()
         } else {

tests/by-util/test_sort.rs

@@ -2960,4 +2960,25 @@ e f 5436 down data path1 path2 path3 path4 path5\n";
         .stdout_is(input);
 }
 
+#[test]
+fn test_consistent_sorting_with_i18n_collate() {
+    // Regression test for issue #11980
+    // Lexicographic fallback sorting for equal sorting keys for 01 and 0_1
+    let expected_output = "0_1\n0_1\n01\n01\n02\n02\n";
+    new_ucmd!()
+        .env("LC_ALL", "en_US.UTF-8")
+        .arg("fix_i18n_collate_inconsistency_1.txt")
+        .arg("fix_i18n_collate_inconsistency_2.txt")
+        .succeeds()
+        .stdout_is(expected_output);
+
+    let expected_output = "01\n01\n02\n02\n0_1\n0_1\n";
+    new_ucmd!()
+        .env("LC_ALL", "C")
+        .arg("fix_i18n_collate_inconsistency_1.txt")
+        .arg("fix_i18n_collate_inconsistency_2.txt")
+        .succeeds()
+        .stdout_is(expected_output);
+}
+
 /* spell-checker: enable */

tests/fixtures/sort/fix_i18n_collate_inconsistency_1.txt

@@ -0,0 +1,3 @@
+01
+0_1
+02

tests/fixtures/sort/fix_i18n_collate_inconsistency_2.txt

@@ -0,0 +1,3 @@
+0_1
+01
+02