To reproduce:
1) Create new project
2) Create Editor folder and a script in there, use the code of first example here: https://docs.unity3d.com/ScriptReference/GenericMenu.AddItem.html
3) After the "GenericMenu menu = new GenericMenu();" add:
menu.allowDuplicateNames = true;
4) Duplicate "menu.AddItem(new GUIContent("MenuItem1"), false, Callback, "item 1");" and "menu.AddItem(new GUIContent("SubMenu/MenuItem3"), false, Callback, "item 3");" right under the corresponding originals
5) Open the newly made custom editor window
6) Right click in it to create the added context menu

Expected: Both duplicate entries are shown
Actual: Two MenuItem1 are shown, but only one MenuItem3

Reproduced in 2018.2.0b11, 2018.2.9f1