Here's Lucy (1968)

S5E11 Lucy and Donny Osmond

Lucy and Kim go to a Donny Osmond concert with their cousin Patricia (Eve Plumb), who is enamored with the 14-year-old star. During the performance, Donny notices Kim in the crowd and instantly falls for her. Kim assumes the gifts and letters are meant for Patricia, only to later find out the truth. When she learns that Donny agreed to appear at her charity event due to the misunderstanding, Kim feels compelled to clarify the situation.