To reproduce:
1. Open attached project ("AnimationCurveTest.zip")
2. Attach the only script to any GameObject
3. Open the curve editor by clickin the curve
4. Add a key to the curve at time=0 (any other value will still exhibit the same behaviour, but this is the most problematic example)
5. Add another key
6. Right click the added key, and select "Edit key"
7. Try to input a value that starts with the time of the first key (in the example where the first key's time is 0, the value would be 0, ..., e.g. 0,5)
Notice that while the key's time value is being input, it becomes the same as the first key's time value, and merges both keys, thus destroying the first one.

Reproduced on: 5.6.0b5, 5.5.0f3
Not reproduced on: 5.4.3f1