Append Query Linked To A Form Problem?

Please can you help me find the issue to my problem that is probably hitting me in the face.

I have an unbound form to which i use an append query to post the data from the fields on the form to a table.
The table is set up very simple and has a primary key which i will never duplicate as you would expect.

Once i have inputted one lot of data into the form the command button runs the query and the data is posted to the table fine. The problem i am having is that the second lot of information i go to append the query refuses to add.
I get the validation rule violations error and it won't post the data.

If i close out of the form, open it again and input the same information the query adds it fine and again the second lot you get the error message.

i had this problem before but for the love of me cannot remember how i resolved it. I have had to keep closing the form after each entry to post the information.

Any help much appreciated.