Plugins can not replace parent cell references with a cell reference of different type
abotWindowsGlow.esp includes something that I didn't know was possible:
Morrowind.esm:
RefNum: 113840 RefID: ex_nord_win_01 Cell: Seyda Neen
abotWindowsGlow.esp:
Refnum: 113840 ID: 'ab01Ex_nord_win_01'
Note ex_nord_win_01 is a Static, but ab01Ex_nord_win_01 is an Activator!
In the original engine this kind of replacement works (though I imagine there might be issues if the replaced object was already in a save-game, or if you attempt to replace types that are not at all intercompatible, e.g. replacing a static mesh by a creature). It's not something I'd recommend doing, but appears to work.
In OpenMW, replacing a reference with another reference of a different type does not work, because the CellStore uses a separate list for each MW-type. Both references get loaded, resulting in a duplicate.
(RM-3085 from redmine: created on 2015-12-15 by scrawl ., , closed on 2016-07-23 by Miroslav Remák, relates #2719 (closed), relates #3471 (closed))