Validates that no name occurs twice. Unnamed elements are ignored; use
require_named() to reject them.
See also
Other structure checks:
require_dim(),
require_has_cols(),
require_length(),
require_length_matches(),
require_length_max(),
require_length_min(),
require_named(),
require_names(),
require_ncol_matches(),
require_ncol_min(),
require_nrow_matches(),
require_nrow_min(),
require_scalar(),
require_sorted()