Just tried creating a fresh project in AC18 from the office template and migrating it forward and I am getting the same result. This is strange since the duplicates all seem to be textures and the loading report seems to indicate that the duplicates are between the office library and AC19's library. While I have a few textures in the office library none of them are what gets listed and there are nowhere near the 254 listed duplicates.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K