[Finder] Fix unexpected duplicate sub path related AppendIterator issue

Bug fix: yes
New feature: no
BC breaks: no
Deprecations: no
Tests pass: yes
Fixes: https://github.com/symfony/symfony/pull/4993#issuecomment-8515845
Todo: -
License of the code: MIT

> @dg:  I am afraid it is not working, see this example http://davidgrudl.com/tmp/appenditerator-finder.zip.