Correct spelling

amended

Correct spelling, explanation: this form is the correct one, because we do not use double m just as it wasn’t doubled in Middle English origin – amenden. There has never been double m in any previous form of amended (base form: amend), so ammended is incorrect. The correct form is amended.

Definition of amended:
verb, past simple and past participle form of amend, to modify, usually to improve, or to add something, for example to a document
The agreement was amended due to the fact that the company’s name had been changed.
Government is planning to have this Act amended.


Incorrect spelling

ammended

Incorrect spelling, explanation: ammended is incorrect, and amended is correct, because the base form of this verb is is amend, which is spelled with one m, just as its Middle English original form (amenden). There is no need to double the m, so the correct form is amended.