Update November 8 2012 This was fixed in both Outlook 2007 and Outlook 2010 by recently released hotfixes.
Outlook 2007 users need to install the Word 2007 hotfix in Description of the Word 2007 hotfix package (Wordconv-x-none.msp, Word-x-none.msp): October 30, 2012
Outlook 2010 users need the Word 2010 hotfix described in Description of the Word 2010 hotfix package (Word-x-none.msp, Kb2428677-x-none.msp): October 30, 2012
Users are complaining that Outlook is adding duplicate extensions to attachments. For example, Outlook is adding .pdf to an existing .pdf file and filename.pdf becomes filename.pdf.pdf. All file types are affected, not just .pdf. Each time the attachment is forwarded, an additional extension may be added.
This is the result of a recent update. The problem updates are KB2687315 (Office 2007) and
KB2553488 (Office 2010). These are updates address critical issues in Microsoft Security Bulletin Summary for October 2012 and patch vulnerabilities in Word for which active exploits.
Until you install the update, one workaround is to drag the attachment into the attachment field of the new email and no extra extension is added. Because the attachment field only shows up after you add an attachment, you'll need to add an attachment, delete it then drag an attachment into the attachment field.
Attachment file extension is duplicated when added to email in Outlook 2007 and Outlook 2010