How to rename Tabs a second time???

Hi, I am trying to customize my implementation and have been struggling with changing the names of the tabs. I have tried several different terms for "Leads" "Prospects", but I can't seem to change the name back to something I have tried before. (If I try this I get an error message that the proposed new tab name is a "duplicate" tab name, even if it is something I had tried before but am not using now. Is there any way to fix this or work around it?