The keyframe boomerang effect is a consequence of using bezier keyframes, or paths that use handles on your path nodes. It’s most prevalent when copying and pasting keyframes with the intention of having the layer hold the same properties for an extended period of time before resuming motion. The issue: The layer doesn’t hold and instead continues motion on a path you didn’t specify. 

‍

‍

Understanding this opens up three ways (and potentially many other options) to fix this pesky issue:

1. Use the pen tool and remove the handle from the keyframe node on the canvas.
2. Toggle frame hold by right clicking on the first of the two repeated keyframes and selecting “Toggle Hold Keyframe.”
3. Adjust handles in speed graph by toggling the speed graph, find the kink in the curve and select the first of the two keyframes and toggle the hold keyframe icon or adjust the handles off of the keyframes.