Fix repeated word typos

[xofbd]

A few typos I found when reading the documention.

[matejak]

This is tricky, because in a way, the "default default" is correct wording. What it wants to say is that e.g. ARG_OPTIONAL_SINGLE accepts a parameter default that is optional, and the default value of that parameter is an empty string. So, the empty string is a default value of the default parameter, i.e. default default.
What do you think?