Prevent or better handle the case where 2 keyframes have the same frame position.

[rewbs]

Currently there's nothing preventing you from having 2 keyframes with the same frame number. This causes errors (with poor error messages) when using some functions like cubic spline. Even without errors the outcome will be difficult to predict.

Simply preventing duplicate keyframes seems like a no-brainer, but it's actually something we might want if we introduce "magic" keyframes that track prompt positions as they are updated. That said, in the absence of such features, blocking dupes is probably the way to go.