I encountered this same bug today.
I started working through some of my recent observations to add annotations, using the keyboard shortcuts to add and to move to the next observation. After adding an annotation I saw the spinning icon indicating the system was working; it took a long time to complete, and sometimes failed to register the annotation.
After adding annotations to about four observations I went back to look at the previous ones, and found that duplicate annotations had been added. Then I moved forward to newer observations and found that annotations had been added before I even got to them!
Moving to the “Your Observations” tab, I found that the duplicate annotations were present in the observations that I had edited, but were no annotations had been saved in the observations that I had not really touched (but showed in the Identify tab). Clear as mud?
In the screen shot below, the only annotation I entered was Life Stage. The rest appeared auto-magically. Only the Life Stage was saved to the observation in this case.