I am opening this bug because duplicity-0.4.3 requires at least pexpect-2.1, but pexpect-0.999 is what is currently emerged since pexpect-2.1 is marked as unstable. Please add the version number to the RDEPEND. Here's the original bug I opened with duplicity where I learned of this version dependency: http://savannah.nongnu.org/bugs/?func=detailitem&item_id=21153 This is also related to Bug 192270.
Fixed in CVS. Thanks and sorry for the inconvenience.