uniqueness of Serial Number

Hi,

I have problems with the uniqueness of serial numbers. Frist of all I have created a material, afterwards I have maintained the serial number profile. I have added the profile to the material. And I have created

serial numbers for this material.

Now I can choose one of the serial number and add it into the material in the sales order. I can create delivery and ivoice and in all documents this serial number is in.

Now, if I create a second sales order I can choose the same serial number like in the first sales order,

but this is wrong.

Perhaps anyone has an idea?