start_compose will never run if `allow_duplicate` is set to `false`

[jharmison-redhat]

Because the list is made up of several lists concatenated together (see here) inside of another list, even if all four lists are empty the outer list will always have a length of 1 (the empty list result of all the concatenation). Because of this, len(dupe_compose) will always equal 1 under all circumstances, causing this check to react as if no compose is necessary - even if no composes exist at the Weldr API at all.

If we suppose that we have potentially four empty lists for compose_queue_new_dupe, compose_queue_run_dupe, etc. the code is equivalent to this:

>>> empty_list_1 = []
>>> empty_list_2 = []
>>> empty_list_3 = []
>>> empty_list_4 = []
>>> empty_lists = [empty_list_1 + empty_list_2 + empty_list_3 + empty_list_4]
>>> empty_lists
[[]]
>>> len(empty_lists)
1