I can tickle myself in several spots on my upper back, and I definitely don't think it feels like someone else is doing it (and don't have trouble predicting it from what I can tell, although I guess I've never been tested on that rigorously). If anything, my expectation of it causes it to be even more overwhelming (it's not something that I enjoy at all honestly regardless of whether it's me or someone else doing it; the best comparison I can make to it is like accidentally shocking yourself when brushing your finger against the metal when plugging something into an outlet, just not as strong and localized to the tickled spot).

On the other hand, I've noticed a fair number of other strange reactions that I sometimes get to sensations that I've occasionally seen other people online mention but have never met anyone else in person who also says they have them. One weird one is that when certain things vibrate in my hand (e.g. some but not all of the cell phones I've had and my Nintendo Switch's "rumble"), my inner ear will feel like it's shaking around, which is very unsettling. It seems to happen on the opposite ear of the hand my phone is being held (or both ears if both hands are touching). I can also sometimes replicate it by tickling parts of my lower face (in which case it happens on the same side as the part of my face touched), which makes me think it's due to some sort of actual physiological response to some extent rather than just some sort of weird resonance). From googling this in the past, there do seem to be other people who experience this, but I couldn't find any sort of documented research on it (presumably due to it being rare and benign enough not to merit anyone's attention).