A couple of month ago i did a bug report about an 'Failed to insert data' error when I tried to copy or import an assessed survey.
A this moment it was difficult to reproduce the problem because I was using several installations and missunderstood what was going wrong.

Now it looks moor evident:
You can't copy an assessed survey on the same LS platform without loosing all the assessment settings du to duplicated IDs. (to reproduce the problem try to import 2 times the attached sample survey)
Example for a survey.lss with:
<id><![CDATA[1312]]></id>

You w'll get after copy:
Error: Failed to insert data
$query
Duplicate entry '1312-en' for key 'PRIMARY'

Is it a bug to fix or a known limitation of LS ?

You should submit that as a bug.

I am not fully aware of the assessments concept but the duplicate id is a bug to me. If such variables are used in the imported survey only they need to be recreated with a different id. If they are used in a global manner the import/copy routine needs to ask if existing assessments should be used or overwritten or recreated.