Skip duplicate repo URLs during update

[HebaruSan]

Problem

A user reported this exception at start-up (partially translated from Chinese):

Unhandled exception:
System.ArgumentException: An item with the same key has already been added.
   在 System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   在 System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   在 CKAN.Repo.<>c__DisplayClass0_0.<UpdateAllRepositories>b__1(Uri url, String filename, Exception error, String etag)
   在 CKAN.NetAsyncDownloader.FileDownloadComplete(Int32 index, Exception error, Boolean canceled, String etag)
   在 CKAN.NetAsyncDownloader.<>c__DisplayClass20_0.<DownloadModule>b__1(Object sender, AsyncCompletedEventArgs args, String etag)
   在 System.Net.WebClient.OnDownloadFileCompleted(AsyncCompletedEventArgs e)
   在 CKAN.ResumingWebClient.OnOpenReadCompleted(OpenReadCompletedEventArgs e)
   在 System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   在 System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   在 System.Threading.ThreadPoolWorkQueue.Dispatch() 

Cause

This callback could throw that exception if you had multiple repos configured to use the same URL:

CKAN/Core/Net/Repo.cs

Lines 56 to 58 in 8390a61

	// Capture repo etags to be set once we're done
	var savedEtags = new Dictionary<Uri, string>();
	downloader.onOneCompleted += (url, filename, error, etag) => savedEtags.Add(url, etag);

Doing that would not make sense, but apparently it happened anyway.

Changes

Now we filter out repo entries with duplicate URLs early in the update process, so we'll only download them once and the exception will not be thrown.

Also some strings in repo updating that I forgot to internationalize in #3645 are now internationalized.

Fixes #3761.

[techman83]

Awesome, LGTM!