don't emit redundant error when checking a named type against itself

src/Psalm/Internal/Type/AssertionReconciler.php

@@ -457,6 +457,9 @@ private static function refine(
             if ($key
                 && $code_location
                 && $new_type->getId() === $existing_var_type->getId()
+                //even if two objects are the same, equality is not guaranteed
+                // example: (ErrorException and TypeError are both Throwable but not equal)
+                && !$new_type->hasNamedObjectType()
                 && !$is_equality
                 && !($original_assertion === 'loaded-class-string' && $old_var_type_string === 'class-string')
                 && (!($statements_analyzer->getSource()->getSource() instanceof TraitAnalyzer)

tests/TypeReconciliation/TypeAlgebraTest.php

@@ -1102,6 +1102,42 @@ function a(int $a, ?int $b = null): void
                         }
                     }'
             ],
+            'ThrowableInstanceOfThrowableMayBeFalse' => [
+                '<?php
+
+                    final class Handler
+                    {
+                        /**
+                         * @var class-string<Throwable>[]
+                         */
+                        private array $dontReport = [];
+
+                        /**
+                         * @param class-string<Throwable> $throwable
+                         */
+                        public function dontReport(string $throwable): void
+                        {
+                            $this->dontReport[] = $throwable;
+                        }
+
+                        public function shouldReport(Throwable $t): bool
+                        {
+                            foreach ($this->dontReport as $tc) {
+                                if ($t instanceof $tc) {
+                                    return false;
+                                }
+                            }
+
+                            return true;
+                        }
+                    }
+
+                    $h = new Handler();
+                    $h->dontReport(RuntimeException::class);
+
+                    $h->shouldReport(new Exception());
+                    $h->shouldReport(new RuntimeException());'
+            ],
         ];
     }