Incoming IPN received email does not match business ... Hacked?

I wondered why someone would send my site a $1 donation yesterday until I noticed in my logs now errors messages of:

This transaction has already been processed, ignored: ...

and the most disturbing:

Incoming IPN received email does not match business email (received grandinp@altern.org, business mymail@thedomain.net)

Obviously there was a hack attempt, but were they successful? And in either case, how do I clear this up? Has he infiltrated my actual paypal account you reckon or just trying to intercept my donations? What should I do?

Any advice appreciated.

Thanks.