Prep before tile-over-tile job.

I have a kitchen floor that now has peel-and-stick tiles on it that are worn in some areas. Other than the wear areas, the tile is flat, smooth, solid and in good shape. I want to re-tile the floor with peel-and-stick tiles and I want to put this layer of tiles over the old tiles. I know it is sometimes better to remove the old tiles first, but this time around I definitely just want to tile over the old tiles.
So, here's my question.
Is there any particular type of floor stripper or surface prep that I should use on the old tiles before laying the new tiles?
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worn
should
Once you get the first edge up using a putty knife. I duct-taped an 8 inch drywall knife on a broom handle and kept whacking at the underside of the tile. Takes time, but it does work.
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Thanks, but I don't want to take up the old tiles (even though I know I probably should). I am going to put the new tiles directly on top of the old tiles. What I was trying to figure out is whether there is some type of floor stripper that will clean the old tiles and not leave any kind of residue so the new tiles will stick better to the old tiles.
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