Singing or playing a duet stands for partnership and the effort of staying in tune with another person. Dream books read harmony in it as a bond that works, and clashing as a relationship out of step.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.