簡單一句中翻英

中翻英內容:
2月將退回部分的多收取的金額，因為合約期是標示著上年10月到明年9月2年期，而非1年，使得收取了過多的收入。

中翻英:We will refund you the over-paid part of the received payment. As the actual contract period is stipulated as two years from October last year to September next year and not one year, some excessive payments have been received.

英文表達:There have been over-payments received because of an misunderstanding of the contract period to be one year, instead of two years from October last year to September next year. Therefore, we will refund you in February the amount of excessive payment.

If I want to express the following simple English, what should I write:
February more than the amount charged will be refunded part of the contract period is marked as a 2-year *, rather than a year, making the charge too much income.
* October last year to September next year

we will refund the over payment in February because the contract period is for 2 years from October 2014 to September 2016 instead of 1 year. Consequently, there are over payment in the last 4 months.