inconsistent results for IBugTarget.searchTasks(has_patch=...)

sample launchpadlib script:

ubuntu = launchpad.distributions['ubuntu']
milestone = ubuntu.active_milestones[6]
print milestone.name

with_patch = milestone.searchTasks(has_patch=True)[:100]
no_patch = milestone.searchTasks(has_patch=False)[:100]

no_patch_set = set(x.self_link for x in no_patch)
with_patch_set = set(x.self_link for x in with_patch)

print "len no patch / set no patch", len(no_patch), len(no_patch_set)
print "len with patch / set with patch", len(with_patch), len(with_patch_set)
print "intersection no/with patch", no_patch_set.intersection(with_patch_set)

The list with_patch contains some duplicate results; the intersection of with_patch_set and no_patch_set is not empty.

This inconsistency occurs too for calls of searchTasks(has_patch="true") on distributions and products.