Why are numbers usually written twice in contracts?

Its done to make any tempering with the document difficult. Maybe one can change the number in one place but it wont be easy reflecting the change in the other representation

Checks (bank drafts) have the transaction amount written twice: once in numerals and once spelled out. The numerals are the "convenience amount" and the words are the "legal amount" in banking parlance. When they don't agree, the legal amount trumps the convenience amount on the theory that it's harder to tamper with the words than the digits. (This applies in the U.S. I don't know if the terminology and rules are the same elsewhere.)

I assume the same rules apply to contracts.