Add test for duplicates in list

List duplicates are not discovered, this test should be a starting point
for a future fix.
May be linked to:
https://github.com/SQiShER/java-object-diff/issues/60
https://github.com/SQiShER/java-object-diff/issues/42
Merge pull request #83 from vladmihalcea/master

Add test for duplicates in list