I propose to detect double postings automatically by the system and perform a minor penalty like a 24h ban of posting content.
There are numerous cases in which users duplicate their own question to get it back into the latest questions feed. This eats up considerable amounts of moderation time and is a clear violation of etiquette. As with the implementation of the quality check before posting, a temporary ban of offending users would encourage to use the encouraged way of posting bounties to add attention to a post.
As duplicate detection is a very resource intensive task, the system would have to work very localized. The detection time frame could be limited to a few days - the exact amount could be determined by evaluating already existing duplicate closes, checking these questions for real exact duplicates (not only by meaning but by Levenshtein distance and performing a statistical analysis of the time, in which 80% of exact duplicates occur (from my current SO experience, I'd say ~72h).
Most duplicates are also posted by the same user, as most users do not go through the hassle of registering a new account for reposting the question, so we could also limit the search to posts by the same user. This lowers the amount of questions to search through to a few at maximum.
The algorithm to use needs more evaluation, as there are several working examples out there. The easiest implementation would be a Levenshtein distance, but it is not the most efficient one.
As we want to positively encourage users to stick with the rules, the offending user should be warned once with showing the consequence of a temporary ban before the question itself is posted, similar to the quality warning of very short posts.
If the question is posted nevertheless and a duplicate is detected, the system should automatically tag the question as reposting for moderation and set a low vote count (around 2 instead of 5) to close it. If the reposting-flag is acknowledged by two positive votes, the new user should be informed and banned for 24h from posting and the reposting should be merged or deleted with the original.