I do admit that a label like "There is already a contributed translation for this string" would be best from user's perspective. But currently there would be a problem with the performance. There is a complex SQL query to obtain all the filtered strings for the translator page (and that one is already slow enough in certain conditions). Contributed translations are not stored in the database until they are accepted, so joining them with the SQL query results would have to be slow one-by-one operation.
But we have more problems with the AMOS storage backend anyway. It had been designed just before Moodle 2.0 was released and it does not cope well with frequent Moodle releases. Plus, when all the plugins were added to it, it made the situation even worse (long story short, we duplicate a lot of things for each branch/version). So it is really the strings storage system that should be rewritten - and then this feature might be added more easily. Maybe.