To reproduce:
1. Open my attached project (SpriteTest.zip), in Project window, select sprite atlas MYATLAS
2. While MYATLAS selected, drag Tree sprite from Project window onto MYATLAS Objects for Packing area (in Inspector window)
3. Repeat step number 2, enter and exit Play mode
4. Enable Inspector window's debug mode to be able to check the contents of Atlas sprites' array

Expected: same sprite is not added to Atlas again - objects for packing shows single sprite added
Actual: same sprite is not added to Atlas again, however objects for packing shows two sprites added (check objects.jpg)

Reproduced in: 2017.1.0b6, 2017.1.0b9, 2017.2.0a3

Note: feature not available in earlier Unity versions