Using spring boot 2.0.4.RELEASE which uses Hibernate 5.2.17.Final the following test fails:
(I created a little test project to showcase the bug)
Eventhough I only added one child to the parent, that child actually appears in the parent's list an equal amount of time as there are grandchildren. 3 in this example.
Further investigating reveals that there are no duplicates in the database, this is purely in the java objects after find. Also changing the List to Set in the parent class "fixes" the problem.