Magento: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '100000001' for key 'UNQ_SALES_FLAT_ORDER_IN

Question

I have installed Magento 1.9.0.1 and I am live since 1 month. The first order of a Client worked without Problem. But now following error message appears when the order should

Answer 1

I quite liked the answer given by Empiro Technologies. I used that answer as inspiration but took a different approach: in my case I had migrated sales_* tables from new database to old copy of database. However I had missed the eav_entity_store table. So I simply updated all of the rows across in that one table, from database to database. That worked fine in my case.