>I have a 'form'(master-detail) that run on network,
>If there are 2 or more run 'form' to input data at the same time,
>then occuring error "...invalid key..." or the key is invalid between tables(master-detail)
>What can i do to solve this problem ?

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I think this is because you are assigning the same primary key in both forms.
What you can do is creating a keynumbers file where you store the keynumber to be used.
Each time you need to create a new record, you look into this file first, get the value out of it, and replace the value with value + 1

In the Tasmanian Traders example is a sample of this in the NewID function.