Only bump error count when we are sure that the diagnostic is not a repetition.

This ensures that if we emit the same diagnostic twice, the error count will
match the real number of errors shown to the user.

Fixes #42106

src/librustc_errors/lib.rs

@@ -588,6 +588,9 @@ impl Handler {
         // one:
         if self.emitted_diagnostics.borrow_mut().insert(diagnostic_hash) {
             self.emitter.borrow_mut().emit(db);
+            if db.is_error() {
+                self.bump_err_count();
+            }
         }
     }
 }