[X][Tithe] Moderate. Publicly name and shame the guilty, then assess a fine to each family for tax-dodging.
[X] Yes.
Not as much as you might think.
It probably started accidentally, remember the fields aren't fenced. Everyone 'knows' where their field is, but theres no actual surveyed and marked boundaries, so its easy to just plow or plant beyond your lines and hey you're friendly neighbors and don't want to make a fuss, as long as they don't try stealing what you planted.
This sort of thing is endemic, and well, after a few years they probably realize they're a bit richer than expected, so why not continue?
You really don't want to make an example of this. You can do it completely by accident and, the worst part is they're actually reducing their yields, the double-planted fields are not being left fallow properly and thus the soil is getting overworked.