The preposition with which you are filling it in

I am currently mired in marking. As you know, I keep note of which mistakes are currently popular. One that I spotted recently was a sort of preposition doubling but with two different prepositions. Let me explain.

So, there are two things English can do. You can either move your preposition (in, in this example) along with your wh-word:

It depends on the newspapers in which you are reading them.

or you can leave it where it is:

It depends on the newspapers which you are reading them in.

It's a stylistic choice nowadays (used to be frowned upon, not so much now, and the first sounds a bit stuffy in casual conversation). Ignore the fact that which probably ought to be that in the second example (incidentally, that's evidence that this which/that rule is a bit daft). English is unusual in allowing this choice: most languages can't 'strand' the preposition and have to move it, as in the first example.

Sometimes, people get halfway through the sentence and forget they moved the preposition, and stick it in at the end as well:

It depends on the newspapers in which you are reading them in. 

Oh well - no problem. Speech error. It happens. Paul McCartney is sometimes said to have sung 'this ever-changing world in which we live in' (though he thinks maybe it was actually 'in which we're living').

But today I read sentences like this from two different students:

It depends on the newspapers with which you are reading them in. 

How exciting! So I think here, the student has left the preposition at the end on purpose, because that's fine, but also felt like there really ought to be something in that space before the which, so stuck in another preposition (with) that sounds OK there. Add it to the list of 'more words = better' mistakes.