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Depending on what's there now and how far does your tile overhangs over the plaster. If more then 3/4 of the tile is over Hardibacker then use mastic for the little section that over hangs the plaster. Personally I scrap the paint and thinset over that. Buy good thinset and addmix, its worth the money.
I like to run builder's paper over as much as I can into the overlapping area under the plaster or drywall. In your case, you might not be able to do that. I always paper the back of the hardibacker for cheap insurance.
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