Developer's note:

This is by design:
When you drag a new FBX file in the Assets folder, we create a new .meta file to go along with it (this is where your import settings live, including your list of clips)
If you have multiple takes in your FBX file, we create an entry in the clip list for it.
Once this file is created, we don't recreate it or overwrite it from changes in the FBX file, because that would overwrite any changes you made.

I understand that you'd like the list of clips to match the list of takes in the FBX file, but this is not the current or the intended behaviour.
If you'd rather have that behaviour, I suggest you delete the .meta file, then re-feed your settings through https://docs.unity3d.com/ScriptReference/AssetPostprocessor.html.

Steps to reproduce:
1. Download the attached models (adventurer and adventurer_test)
2. Import adventurer fbx
3. Set the model's mask to "Create from this model"
4. Import fbx from adventurer_test to overwrite previous model

Expected result: the animation clips from the newly imported model are now in the list
Actual result: animation clips from the previous model are not updated

Reproduced on 5.5.6f1, 5.6.5p2, 2017.1.3p2, 2017.2.2f1, 2017.3.1p4, 2018.1.0b11, 2018.2.0a5