eliminate duplicate multideps from generated module files

[mboisson]

When combining multi_deps with iterative builds, it generates a list with duplicates, for example:

   /cvmfs/soft.computecanada.ca/easybuild/modules/2023/x86-64-v3/Compiler/gcccore/nlopt/2.7.1.lua:
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
help([[
Description
===========
NLopt is a free/open-source library for nonlinear optimization,
 providing a common interface for a number of different free optimization routines
 available online as well as original implementations of various other algorithms.


More information
================
 - Homepage: http://ab-initio.mit.edu/wiki/index.php/NLopt


Compatible modules
==================
This module is compatible with the following modules, one of each line is required:
* python/3.10, python/3.10, python/3.10, python/3.10, python/3.11, python/3.11, python/3.11, python/3.11
]])

This commit fixes that.