How to reproduce:
1. Open the project attached by the user (SpriteAnimation.zip)
2. Open the "SampleScene" Scene
3. Open the Animation Window, select the "Test" GameObject in the Hierarchy
4. Open the "Test" Folder, toggle Recording in the Animation Window
5. Select every sprite in the "Test" Folder and drag and drop them to the Animation Timeline
6. Notice how the 17th sprite in the timeline is a duplicate of the 16th sprite

Expected result: The 16th and 17th sprite in the timeline are different, like in the "Test" folder
Actual result: The 17th sprite in the timeline is a duplicate of the 16th sprite

Reproducible with: 2017.4.0f1, 2017.4.13f1, 2018.1.9f2, 2018.2.11f1
Not reproducible with: 2018.3.0a1, 2018.3.0b5, 2019.1.0a4

Notes: I have found that changing the sample size after the import to the timeline, the problem fixes itself