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 had been spending quite a bit of time trying to diagnose the same error I had, I couldn't submit orders through paypal with one store view but I could with my other views. After viewing this post I realized that my increment_last_id was the culprit! I just recently imported old customer info/orders from oscommerce->magento using cart2cart and it placed all my orders into my default store view without updating the increment_last_id. After updating that int to the last imported order number I could place orders again!