1. A concise explanation of the problem you're experiencing.
Adding entities (or any kind of Cesium object with a definitionChanged object) to an Entity as a non-standard key causes a listener to be added to it, and not removed.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
We are trying to dynamically add and remove objects from an Entity, or assign things such as SampledPositionProperties, however the process of doing this causes listeners to build up.
4. The Cesium version you're using, your operating system and browser.
1.36, OSX 10.12. Chrome 60