Do not re-alias SerializationException (#154)

Author: danog

lib/Sync/functions.php

@@ -4,8 +4,10 @@
 
 use Amp\Serialization\SerializationException as SerializerException;
 
-// Alias must be defined in an always-loaded file as catch blocks do not trigger the autoloader.
-\class_alias(SerializerException::class, SerializationException::class);
+if (!\class_exists(SerializationException::class)) {
+    // Alias must be defined in an always-loaded file as catch blocks do not trigger the autoloader.
+    \class_alias(SerializerException::class, SerializationException::class);
+}
 
 /**
  * @param \Throwable $exception