Issue renaming Source File in Dexterity

I mistakenly renamed a Form in Dexterity and am now unable to rename it back to the original value. When I enter the original Source File name in the "Edit Source File Names" screen, Dexterity changes it to append "0" to the value. What do I need to change to have Dexterity allow the original Source File name value to be reverted back?

Thanks,

Shelfrog