I  just downloaded  and installed your example from https://github.com/driftingruby/042-fullcalendar and seems it has the same issue I have encountered.

To reproduce the issue:

1.  create an new event or update (either edit or drag&drop) an existing event. (everything is good so far)

2 . click on next month button, then click to go back to current month

3. you will see two duplicate events you just created/updated

Any clue?