Part of the reason for the question is that the way the lose-track rule is stated is a little ambiguous:
In rare circumstances an effect may try to move a card that isn't where that effect expects the card to be. In those cases the card does not move - the effect has "lost track" of the card.... Things lose track of a card if something moves it, if it's the top card of a deck and gets covered up, or if it's the top card of a discard pile and gets covered up.
There are two ways to read this. One reading, which is the one Donald's ruling above is based on, is that the first sentences are the
motivation for the lose-track rule, and the last sentence is the
definition of when lose-track happens—for a card to be lost track of, it's sufficient for it to have been moved or covered up.
The alternate reading, which is apparently not correct but is also compatible with the way the paragraph is written, is that the
first sentences are the definition, and the last sentence is a listing of the situations in which the definition can apply. Under this reading, for a card to be lost track of, it's
necessary for it to have been moved or covered up, but not sufficient; it also has to be "not where that effect expects the card to be".
Cases like the one Gendo alludes to, where you've got a Duplicate on your Tavern mat already, play another Duplicate with Prince, and then call
one of them that turn and discard it from play, convinces me that the former interpretation is
probably the preferable one, since how do you know whether the Duplicate you discarded from play is the same one the Prince played.