ListElementNeighbors returns elements at all found positions on all parents

One of the bugs discovered from looking into requests#1:

Assuming the following structure:

• Folder A
  • Page A1
  • Page X
  • Page A3
  • Page A4
  • Page A5
  • Page A6
• Folder B
  • Page B1
  • Page B2
  • Page B3
  • Page B4
  • Page X
  • Page B6

Page X is in both folders; ListElementNeighbors for X should, in theory, return X twice with different parents (X in A, X in B) as well as the previous and next page on both parents: A1 and A3, and B4 and B6. In reality, it currently would return all of the above pages! There is a missing JOIN…ON condition in the SQL query.