In case of assigning material no for a material master, i have selected external no arrangment.
How does the system check if the external nos are repeated? What is the system setting required?
When we develop any program, first we validate the input data i.e. number field should not contain alphabatic letter or date field should not have alpha letter. now for material master, the basic data i.e. material number is updated in MARA table so system must have defined the logic that when you input external material number it will check in this basic table and if number is available then it will throw error.
This is just my guess, but you can explore it with the help of technical expert.
If you are assigning external number assignment to Material master, system will not allow you to creat two materials with same material no.
Eg. If you have already created a material no. sa1, and next time when ever you use the same material no. while creating a new material, system will not allow you.