detect and deal with indextarget duplicates

Multiple targets downloading the same file is bad™ as it leads us to all
sorts of problems like the acquire system breaking or simply a problem
of which settings to use for them. Beside that this is most likely a
mistake and silently ignoring it doesn't help the user realizing his
mistake…

On the other hand, we have 'duplicates' which are 'created' by how we
create indextargets, so we have to prevent those from being created to
but do not emit a warning for them as this is an implementation detail.

And then, there is the absolute and most likely user mistake: Having the
same target(s) activated in multiple entries.

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