Thinset over mastic over durock seams in shower

I am putting tile over durock in my new shower installation and the tile guy rightly recommended thinset. However, when I installed the durock I had different information and floated the seams with some mastic I had laying around. So, do I have to remove the mastic before I use the thinset, guessing the answer is yes, but confirmation is always nice.
Thanks.
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Barker9 writes:

No. Did you use fiberglass joint tape?
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