Current behavior when retrieving children ordered by column with a duplicate order column is to skip the duplicate child.
This leads to silent errors.
This could be resolved by:
Throw an error when duplicate order column is found
Or better yet, a duplicate order column value means that order of those duplicate elements is not important and thus should be added to collection in undefined order and continue where it left with next order column value.
Database constraint to not allow duplicate parent/order pairs.