Relisting items

[outgrown18]

I generally manually relist items (in bulk) so I can review any changes I want to make. This has caused problems with doubling an item. There doesn't seem to be much rhyme or reason behind when and item ends unsold whether it shows up as sell similar or relist.  When I go to the unsold tab I mark the items I wish to relist.  Some say relist and other say sell simiar.  I always go to the top box and click relist.  This tends to create an additional item number.  If I don't catch it visually, I can relist the unsold item twice or more. Am I missing something that would help alleviate this problem?