Pasting keyframes removes the existing keyframes

When pasting multiple keyframes into a property with already existing keyframes, the old keyframes are removed. This is a case for both when:
- pasting into the same property (this is a regression) and when
- pasting into another new property (new feature so it's not a regression).

I'd expect that the new keyframes are added on top of the old keyframes, not that they replace them.

Example:

There are keyframes on frame:
1..45..8..

I'm pasting keyframes:
1.....7..10

result:
- keyframe 1 is changing the value
- keyframes 4,5,8 are removed
- keyframes 7,10 are added

Expected result:
1..45..78.10