duplicate externals cause update error -- thoughts?

Hi,

I've created a check during 'svn propset' that refuses to save svn:externals
props that contain the same target more than once. The result is an error,
same as with any other externals format violations. (#4227)

Turns out the same parsing code is (obviously) run during update. So in the
externals phase of the update, the update will bail if the server sends an
svn:externals with duplicate targets.

Is that bad? Is that good?

The standard update will complete. Only updates of the externals items will
interrupt. (And the other entries of that svn:externals prop will not be
handled until a later 'svn up'.)

Subsequently, the prop can be edited and the duplicate removed; an update
after that will complete the update properly. In short, it works just like
any other svn:externals format error that happens to come in from the repos.

At first I thought I would need to disable the dup check for update, i.e.
only enable it for the actual propset/propedit invocations. But now that I
realized the error is recoverable, I think an error message is not that bad
at all. Like this you are told that your external will be ambiguous and it
will not update externals unless you fixed it.

Another possibility is to simply print a warning and not bail...

(All of this is just about duplicates within one single svn:externals prop)

I'm not sure which way to go, so thanks for any thoughts.
~Neels