Multiplicity-free representations of the principal A1-subgroup in a simple algebraic group

Let G be a simple algebraic group defined over an algebraically closed field k of characteristic p>0. Here we classify all irreducible kG-modules for which the principal A1 has no repeated composition factors, extending the work of Liebeck-Seitz-Testerman which treated the same question when k is replaced by an algebraically closed field of characteristic zero.