Possible Clues:

• Unnecessary person, on a double date
• Double-date tagalong
• Unnecessary person
• Misfit
• Odd man with two couples
• Superfluous person
• Double-date extra?
• Person with two couples
• *Double-date extra
• Extra person on a double date ... who's depicted by the last O in 17-, 24-, 39- and 52-Across?