A selectively labelled compound may be multiply labelled when in the unmodified compound there is more than one atom of the same element at the position where the isotopic modification occurs, e.g.
H in
CH4, or there are several atoms of the same element at different positions where the isotopic modification occurs, e.g.
C in
C4H8O.
Sources:
Blue Book, p. 515 (https://www.acdlabs.com/iupac/nomenclature/)
PAC, 1981, 53, 1887. 'Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds' on page 1893 (https://doi.org/10.1351/pac198153101887)