Allow union lambdas to have duplicates

crates/compiler/types/src/subs.rs

@@ -2650,11 +2650,14 @@ impl<L> UnionLabels<L>
 where
     L: Label + Ord,
 {
-    pub fn is_sorted_no_duplicates(&self, subs: &Subs) -> bool {
+    /// Checks if the union of labels is sorted by label.
+    /// Duplicates *are* admitted, since this represents a lambda set, in which we may have
+    /// duplicate lambda captures, if those lambda captures have different representations!
+    pub fn is_sorted(&self, subs: &Subs) -> bool {
         let mut iter = self.iter_from_subs(subs).peekable();
         while let Some((before, _)) = iter.next() {
             if let Some((after, _)) = iter.peek() {
-                if before >= after {
+                if before > after {
                     return false;
                 }
             }