lconvert qt_xx.ts qt_xx.po generates non-validating .po file

lconverting the Qt provided qt_XX.ts files from the translations/ subdir generates a .po file with duplicate strings, which does not validate anymore.

$ LC_ALL=C msgfmt -o /dev/null libqt4_ar.po
libqt4_ar.po:8853: duplicate message definition...
libqt4_ar.po:157: ...this is the location of the first definition
msgfmt: found 223 fatal errors

the reason for that seems to be that the context descriptions in linguist .ts files are lost in the .po conversion step.