First name duplicated on submit to SagePay

When forwarded to SagePay, the first name is duplicated. For example, if I enter Bob Bobson into my site's form, after being forwarded to SagePay it says 'BobBob Bobson.' After contacting SagePay, they sent me the following log from their test. I tried looking through the module files for the billingfirstnames and deliveryfirstnames variables, but I couldn't find them:

09/04/2014 04:22:56 vpsprotocol=3.0 txtype=PAYMENT paymenttype=PAYMENT vendor=tdspares vendorname= accounttype= username= website= vendordata= vendortxcode=41545_403490102 amount=13.94 currency=GBP description=Your order from TDSpares vspterminal= address= postcode= billingpostcode=A11 1AA billingaddress= deliveryaddress= deliverypostcode=A11 1AA contactnumber= contactfax= customeremail=person@place.com basket=2:blb1097:1.00:8.29:1.66:9.95:9.95:Standard shipping (UK):1.00:3.99:0.00:3.99:3.99 allowgiftaid=0 applyavscv2=0 billingsurname=Test billingfirstnames=SagePaySagePay billingaddress1=North Park billingaddress2= billingcity=Newcastle upon Tyne billingcountry=GB billingstate= billingphone= deliverysurname=Test deliveryfirstnames=SagePaySagePay deliveryaddress1=North Park deliveryaddress2= deliverycity=Newcastle upon Tyne deliverycountry=GB deliverystate= deliveryphone= referrerid= billingagreement= token= storetoken= basketxml= customerxml= surchargexml= firecipientacctnumber= firecipientdob=