Unable to execute dex: Multiple dex files define Lcom/myapp/R$array;

I had the same problem, quite weird because it was happening only when using Eclipse (but it was OK with Ant).
This is how I fixed it:

• Right click on the Project Name
• Select Build Path -> Configure Build Path

• In Java Build Path, go to the tab Order and Export

• Uncheck your .jar library

Only sometimes:
In Order and Export tab I did not have any jar library there, so I have unchecked Android Private Libraries item. Now my project is running.