Accept duplicate entries in INI config files

[DimitriPapadopoulos]

Rationale:

1. The -D command line option can be specified multiple times. It makes sense to be able to specify the respective config file option dictionary multiple times too.
2. While specifying an option multiple times might be an error, setting strict to True catches duplicates only within a single INI file, not across multiple INI files. If we are serious about handling duplicate options, we should therefore handle that manually, distinguishing between options that can be duplicated or not.

Fixes #2727.

[larsoner]

Worth adding a test that fails on main but passes here?

[DimitriPapadopoulos]

Indeed, we could add a test with multiple dictionary lines in a config file.

[DimitriPapadopoulos]

Besides, it doesn't even work because the underlying data structure is still a dict: in case of duplicate keys, the new value overwrites the old value, which makes sense but is not what we would like here. I will try a few things, such as:

dictionary = (dictionary1.txt, dictionary2.txt)

[yarikoptic]

an alternative could be specification via , but also there is #1980 which offers to list on separate lines - IMHO might be a better choice

[DimitriPapadopoulos]

#1980 does not just concatenate lines as claimed, instead it ",".join(lines). This will work with option -S/--skip which expects a “comma-separated list of files to skip”, but not with option -D/--dictionary. Therefore, it is not an alternative, we would have to modify option -D/--dictionary to accept a “comma-separated list of dictionaries” anyway.

[DimitriPapadopoulos]

Replaced by #2767, a better alternative (see #2727 (comment)).