What is a Double Spend?
As described in the "How Addresses Work" section above, since sending a transaction will move your entire balance to a completely new address, if you have more than one pending transaction only one can be eventually confirmed and the resulting balance is sent to your next wallet address. This means that the other pending transactions are now sent from an address that has a balance of 0 IOTA, and thus none of these pending transactions can ever be confirmed.
Troubleshooting: Simply create a new transaction to the intended recipient whose transaction was double spent, and be sure to avoid sending a transactions while a previous transaction is still pending. Only send a new transaction when your previous sent transaction has been confirmed.