Runs fail with duplicate key error

When running DEXTER after some time the run fails with the below message:

System.AggregateException: One or more errors occurred. (An item with the same key has already been added. Key: cproxy)
---> System.ArgumentException: An item with the same key has already been added. Key: cproxy

Using the same job file with a different controller works as expected

Start by reproducing the failure by running DEXTER with the reported job file after some time, recording the controller and the cproxy key involved. Compare the failing controller with the one that works; done means the duplicate-key exception no longer occurs and the run completes successfully.

Written by the indexing model from the issue text.