I just completed my first group dance course and started attending my first few social dances. At the most recent dance I encountered the situation (more than once!) where a second man asked me to dance right after I had accepted the invitation of a first man.

In both cases the first man was not holding my hand, either because we were moving out from sitting behind a table and toward the dance floor or because we were standing on the edge of the dance floor chatting while waiting for the music to start.

What is the best reply to the second man? Also, any help with understanding the social cues and etiquette related to this situation would be helpful, as it was not covered explicitly in dance class.

Traditionally it is the man that answers with something like "sorry she just accepted a dance with me". Now if its a "rescue dance" then something like
"oh I forgot I promise this dance to you" to the second man and "sorry" to the first.