Is it ethically problematic for an internal candidate to attend the job talk of a candidate for the same job?
First, as I inquired about in the comments, most job interviews have both public and private aspects.  Public means that they are open to the entire department (or maybe, and probably in some formal sense are, open to the entire university community): e.g.  there should be at least one job talk, and most often this talk will be advertised on the calendar like any other talk, sometimes called a "special colloquium talk" or something like that.  That makes it a departmental event.  As a department member you certainly have the right to go.  There are also usually some private "interview" portions: this is several faculty members asking questions of the candidate, either one-on-one or in groups (or at meals; even lunches and dinners are part interview, really).  These are usually not public events; rather they are activities on the part of the faculty search committee, though in some contexts, e.g. at a small liberal arts college (SLAC) students and other non-faculty members may play an auxiliary role.  Certainly as a candidate for a job you can't be part of the search committee to hire into that job: that is the Cadillac of conflicts of interest!  So you should try to distinguish in your mind between "search committee activities" and "public events" and be sure not to attend the former.  (As usual, if you are in doubt, ask.)

Now I want to make a few comments on StrongBad's (good, I upvoted it) answer.


  One of the jobs of internal candidates, like all member of a department, is to help recruit the individuals being interviewed such that whoever is given the offer, hopefully you, is more likely to accept the offer.


At least at a large research university, it probably not the case that all department members are involved in recruiting for faculty jobs (either temporary or tenure-track).  For instance students are usually not involved at all other than maybe wandering into the job talk (though at a SLAC they might be), and temporary faculty are not on search committees for permanent faculty.  So I don't necessarily agree with the above sentence.  Moreover being involved in "recruiting" for a position one has applied for again sounds like a whopper of a conflict of interest.


  When you are interviewing after an external candidate, the thing to remember is that in most cases watching what someone else does is not going to help you. One case where there could be a definite advantage is if the search committee has some fixed interview questions and you attend an event that gives you access to these prior to your interview.


Yes, I completely agree with that, and it answers one of the OP's main questions.  Most academic interviews I know are not sufficiently formalized that watching someone else's in advance would help you with yours.  Most questions asked in academic interviews are not content questions or "gotcha" interview questions: they are questions about you.  I also think that a job talk is a strange place for asking pointed interview questions.  

So here is my answer: being an internal candidate for a job is potentially awkward enough so that you should seek some guidance about what to do.  If the job talk and the interview aspects are not separated sufficiently clearly in your department, then I think the ethically correct thing to do is just stay away from the whole thing (but tell the faculty in advance that you are planning to do this, in the unlikely event that they have other plans).  If the question really is attending another job candidate's talk: I see no ethical problem with it, and if you're in a department where it really is expected that faculty in your position attend all such departmental events (again I'm thinking of a small department) then maybe you should go.  It is an academic talk after all and you might learn something.  However I think that you should really (forgive my colorful language) shut the hell up as an audience member in this situation.  Interacting with another candidate in any active way is also a huge conflict of interest.  If you have a sincere academic question, of course you can find a way to communicate your question to the candidate later on.  

I don't see how multiple internal candidates makes much difference.  I guess that if I knew the other candidate very well and was very friendly with her, I would be more inclined to come to her talk rather than skip it...and especially I would be more inclined to ask what her preference is.  I would not do that for an external candidate because it's a potentially loaded question that they shouldn't have to deal with.