Mishnah Megillah 1:4 states:
קראו את המגילה באדר הראשון ונתעברה השנה, קורין אותה באדר השני. אין בין אדר הראשון לאדר השני אלא קריאת המגילה ומתנות לאביונים .
If the Megillah was read in first of Adar, and the year was subsequently intercalated, we read it in second of Adar. There is no difference between the first Adar and the second Adar except the reading of the Megillah and alms to the poor.
The above Mishnah confuses me, somewhat. The beginning says If you read it in Adar 1 and an extra Adar occurred, you have to read it again. The end of it says, "There is no difference between the 1st and 2nd Adar except for Megillah reading." That implies that we aren't supposed to read in one of the Adars. How can you undo what you already read in the 1st Adar if you thought that it would not be a leap year? (It says that afterwards they added an extra month.) So, if you read the Megilla already, and the Mishna tells you that you have to do it again, why does it say there's no "difference ... except for Megilla reading"?