overlapping keyframes can be highlight any color

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Yes, it's correct. This blinky (or colored if overwriting another keyframe) keyframe can help to understand accidentally overwritting a keyframe. This will be helpfull for Ae users.

Yes, at least some indication that replacement has happend, so you can undo your way out of it, if that's not what you intended. Same thing for markers too.

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you sayed;

"If we drag a keyframe on top of another keyframe, the other keyframe is replaced."

that's my point.

When you drag a keyfame to anothers and keep it top, keyfreme can blink red color or green. It's like a warning for "you are replacing it now!".  Than we can understand it's change or not or close to another. 
This basic idea can save time whitout zoom in and try to match keyframes.

If we drag a keyframe on top of another keyframe, the other keyframe is replaced.  There is no "overlapping".

To better see this behanvior, click on the Timeline tab pop-up menu (also called hamberger menu) and change "Use Keyframe Icons" to "Use Keyframe Indicies". 

If keyframe 3 is dragged on top of keyframe 2, keyframe 2 is replaced with keyframe 3 and keyframe 3 becomes keyframe 2.

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i record another video for example;
https://we.tl/t-3SJ8QXPnBs

my idea a suggestion to make things easier.

The blinking (color blinking or something as guide)  keyframe can be guide for another keyframe changes. 

A valid idea, though probably not as trivial as it may look. You still need to have criteria how the keyframes are colored or how they are drawn on top of each other.

Mylenium