Not sure if this is a bug or not but....
I only have Tinkering II, but I can already repair my Tinkertool with another Tinkertool - as stated in Tinkering III below.
"With Tinkering III, the speed of tool repair doubles yet again. Getting speedy! But also, you can use a Tinkertool to repair itself. Talk about snakes swallowing tails ..."