This paper puts forward a hitherto unnoticed constraint imposed on natural language syntax, which will be called *{t, t}. This constraint effectively bans syntactic objects (SOs) of the form {t, t}, where the two t's represent traces (copies) created by Internal Merge (IM) (cf. the copy theory of movement; Chomsky 1993). I will first argue that *{t, t} can be straightforwardly deduced from the interplay of Chomsky's (2013) labeling algorithm (LA) and the principle of Full Interpretation (FI). I will then show that the effect of *{t, t} can be observed in a variety of phenomena, including predicate-fronting, copular sentences, criterial freezing (Rizzi, 2006, 2007), and head-movement.