Full Idea
The argument for unshareable properties (the Reduplication Argument) suggests the danger of reduplication of Berkeley; the argument for incompatible properties (Multiple Occupancy) says Berkeley and Hume could be in the same possible object.
Gist of Idea
Essentialism must avoid both reduplication of essences, and multiple occupancy by essences
Source
Penelope Mackie (How Things Might Have Been [2006], 2.8)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.34
A Reaction
These are her arguments in favour of essential properties being necessarily incompatible between objects. Whatever the answer, it must allow essences for indistinguishables like electrons. 'Incompatible' points towards a haecceity.