Target language validation: Some errors are reported multiple times

[petervdonovan]

Example:

In this case, it happens because the same line appears in multiple places in multiple different generated files. The file name and line number information from each of the different diagnostic messages should be kept, but it would be nice if the information could be consolidated.

If it is too hard to combine the different messages, we could use a similarity score (or a rule that simply ignores the file and position given in [] at the end) to determine whether to show the additional messages. It should be acceptable to hide messages because all errors/warnings should be resolved eventually, and when they are resolved, any that remain should become visible.