Take for example this question. This user knows they are cross posting and the question is word for word the same as the one on english.se. I wanted to say something, but I figured someone else would have already since he admits to cross posting. Is this allowed? The consensus seems to be the same on both sites.
Cross posting is not to be encouraged.
The same question can be asked on multiple sites, but it should be tailored to each site's audience. It's the rare question that can be asked "as is" on multiple sites.
The problem here is that the OP was encouraged to ask by comments such as this:
The correct approach should have been for the users on English to flag the post for moderator attention so that it could be migrated.
I see that it's now got identical answers on both sites - hardly useful.
I think the question should have been migrated instead of cross-posted, but that said, cross-posting is discouraged but not against the rules
There's nothing wrong with posting the same question on multiple sites - as long as it's tailored for each site and not a cut and paste repost.
Each site is unique. It has it's own community and own guidelines for what is on and off topic. It would be a very rare question indeed that was on topic on more than one site if posted without modification for that site.
The answer given by one site could be different than the answer given by another site. For example, an English professor might give a different answer to that question than a programmer would. In addition, I wouldn't expect an English professor to know programming terms.