They (singular) has been used since at least Shakespeare, so every single student in an English-speaking country has learned how to use it correctly; including how to format sentences using singular they. 'The firefighter rescued a puppy from a burning building; they were really lucky they spotted the puppy in time.' In any sentence where two pronouns are the same, you'd replace one or the other (preferably the latter though the 'rules' on this are stupidly complex). Alex was drinking Jim's coffee. He should really buy ~~him~~Jim a replacement.