I mean, How to know ECDSA has collisions like that?
I don't think Pollard's Kangaroo will work against ECDSA sigs because there is a SHA512 hash of the message bytes which forms a second line of defence against brute-force.
So even if you cook up a Kangaroo iteration that takes you from R,S to the origional message, it's still hashed, so you'd have to find a different way around that.
Sure you can: R of the signature is the X coordinate of the curve point k*G. So you can use kangaroo to search for R and -R.