07:38 Nov 20, 2009 

Couldn't have said it better myself. (And, indeed, I didn't...)

23:03 Nov 19, 2009 

Every guaranty/ee involves a commitment, but the converse does not hold. It is not redundant to pair 'commitment' with an attributive noun to indicate the nature of the commitment.
Every contract involves a promise, but not all promises are contracts.

18:42 Nov 19, 2009 

...but I do know what "tautology" means. My point is that it seems clear to me that the two words are used (rightly or wrongly) to mean two different things. "Guarantee" is being used to mean the actual, tangible document that the guarantor signs, while "commitment" is used to mean the (intangible) undertaking the guarantor is giving by signing this document.

FWIW, "guarantee commitment" gets c. 18k Google hits. I do not for one moment consider Google to be the be all and end all, but it can be a useful indicator of the currency of a given expression.