To reproduce:

1. Open the project, attached by tester (MenuItemBug.zip)
2. Open the MenuItemTest script
3. Add [MenuItem ("\\//")] to the Call method
4. Go back to Unity and observe the top menu
5. Change MenuItem's item name to "Item1/SubItem//"
6. Repeat 4
7. Change MenuItem's item name to "Item2/SubItem//"
8. Repeat 4

Expected: changing the item's name in the MenuItem attribute modifies the item in the top menu

Reproduced in 5.5.0b9, 5.5.4f1, 5.6.0f2, 5.6.1p2, 2017.1.0b8, 2017.2.0a2
Did not reproduce in 5.4.5p2, 5.5.0b8
Regression introduced in 5.5.0b9

Workaround: issue is fixed after restarting Unity or not using "//" in the item name

Fixed in 2018.1.0a1