It happens sometimes to one of my friends and I: she will accompany me on a song, but then, a singer herself, not get asked to do one. We are not a formal duo either. We have got wise to this, over the years, and if they try to move on to the next person after us, I will usually say, "What about L? Give her a turn". While I appreciate time constraints in a session, neither of us sing particularly long songs, tho there is always some totally insensitive someone who will launch into a 40-verse dreich ballad just when time is running out and half a dozen or so people are still hoping to get a song in!