Lune Spinebot's answer is correct. The slot name of a skeleton must always be unique, so if a slot with the same name exists, a number is automatically added. However, the uniqueness of the slot name is determined by including the folder name, so slots with the same name can exist at the same time as long as they are moved to different folders.
For example, in the following video, a slot named clipping
is moved to another skeleton, but in the other skeleton, the slot with the same name is contained in a folder called Clipping
, so in this case each name is considered unique and no number is added:
Note that the feature that allows you to create folders for slots was added in 4.2, so it is not available in older versions.
I hope this answer is helpful.