Doubly-filled COMP filter

Doubly-filled COMP Filter is a filter which excludes the co-occurrence of a wh-phrase and a complementizer in a COMP-position, as in (i).

(i) I wonder who (*that/*whether) she saw

Examples like these have been accounted for by assuming that complementizer deletion must obtain so as to satisfy this filter. The introduction of the CP necessitates an alternative formulation of this filter. The problem of (i) is now the obligatory deletion of COMP in English.

Link
Utrecht Lexicon of Linguistics