Prohibit adding duplicate Items

Probably a redirect to the edit view would be better?
Add test case test_unable_to_add_duplicate
Use ExistingUserTest.create_example_item() instead of duplicated code
Add new tags to duplicate
Merge branch 'Prohibit-duplicate' of https://github.com/enkore/pypo into enkore-Prohibit-duplicate
New tags from adding a duplicate are searchable
Merge pull request #5 from enkore/Prohibit-duplicate

Prohibit adding duplicate Items and instead adding
all tags of the duplicate to the original