Check for duplicate pixels IDs when adding to list.

Occasionally during testing a CLI import produces duplicate pixels IDs which then causes a test to fail:

http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-integration-python/88/testReport/test.integration.test_chgrp/TestChgrp/testChgrpImportedImage/

This is probably down to a glitch on `stdout` on the node running the integration tests rather than a fault in the import code. This commit should ensure that duplicates are not present.

As this was a very intermittent fail the test should be that a failure like this doesn't occur again. Equally this commit shouldn't break any other tests.