GAE deferred task retried due to “instance unavailable” despite having already succeeded

Question

In our GAE application, we occasionally see TombstonedTaskError errors in deferred tasks due to named tasks being submitted multiple times with identical names. These tasks seem

Answer 1

After being in touch with Google support, it's emerged that the reason is that very rarely, deferred tasks can be executed multiple times by GAE. This means that tasks have to be implemented such that they can safely be executed multiple times without any errors, which was not the case for us (since we assumed that tasks would only be executed once).