Inventory Scan - Monitors showing that do not exist

I am needing to get an inventory of all the monitors "attached" to the computer.  In one example,  the user has two NEC monitors connected but it is showing 4 monitors, 

0 = Syncmaster with serial number

1 =  Syncmaster with serial number

2 = NEC MultiSync with serial number

3 = NEC MultiSync with serial number

I know that the monitor numbers 2 and 3 are correct.  Where did 0 and 1 monitor come from????

Thanks

Cathy