I seems that uiex tool processes tags within <child internal-child> twice, therefore it writes the same entry twice into the en-US.sdf template.
List of affected files:
Caolan McNamara committed a patch related to this issue.
It has been pushed to "master":
Resolves: fdo#57107 processing translatable child nodes twice
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
that seems to work