…nt, the queue would dedup the element itself (as it is stored in a map). However, it would duplicate the children reference for the parent (as this is a list).

Fix:
• check if element to be added is already present in the queue and NoOp in this case.
• extended goDoc on return value

updated mempool Queue method Attach:
• added consistency check for partially overlapping queues
• extended godoc
• included unit test