This phenomenon occurs when the obverse and reverse dies come together in the coining press when there is no silver planchet between the dies ... and portions of the design from one die are transferred to the opposing die ... similar to when wet newsprint ink from one page transfers to the opposite page. Strikes that are subsequentially struck after these dies have clashed will sometimes have this "ghost" effect that you have described. Depending upon how hard these clashed dies came together in the coining press without the silver planchet in between, will determine how pronounced an image will be seen on the finished product.
Just a speculative guess on my part ... which may or may not be accurate.
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