Do you need to know what philosophy is to study it?
I think it perfectly possible to be exercised by questions, to be involved in topics, which are (or are generally classified as) philosophical without knowing that they are philosophical. For instance, a real one in my own case, when I was studying history many years ago I started to wonder how one could understand the past since it didn't exist. I struggled with this question, which is a philosophical question. But I had no idea of what philosophy was or that this was indeed a philosophical question. Again, I was puzzled by questions such as : If I survived death, what would this 'I' be ? A transformed body, a soul (whatever that was), my 'self' (& what was that?).
Anyone is free to doubt my word but I had no idea that there was a subject or inquiry, 'philosophy', to which such questions belonged. Light dawned only when I took political philosophy as a part of my course and connexions with ep & met & logic & ethics drew themselves out in my perplexed little head (or 'mind' if you're a dualist).
'You don't need to know if you are a philosopher, only how the questions you face have been asked and answered before?'
Three points :
Locke's epistemology is important if only to show us in detail why that road leads to destruction. If he hadn't trod it, someone else would have had to. (Roy Mash, 'How Important for Philosophers is the History of Philosophy?', History and Theory, Vol. 26, No. 3 (Oct., 1987), pp. 287-299: 295.)
On this approach you may not need but it is helpful to know 'how the questions you face have been asked and answered before' since this knowledge saves you from following a false or at least unpromising track. You needn't make certain mistakes yourself; somebody else has already made them for you.
2. Problem solved
it has all been said? Or at least everything of consequence? If "nothing can be
said which has not been said already," then is history of philosophy all that is
left to us? Are we to be footnoters and nothing more? Does progress in philosophy consist merely in finding new and interesting ways to cross-index the past? (Mash : 295.)
This is too extreme, too unlikely but there are cases where problems are puzzled over to which either the answer or an interesting part of the answer, or the conceptual means towards an answer, are already to hand in the history of philosophy. Tentatively I would cite the instance of Franz Brentano's work on intentionality which, 'rediscovered' in the 1960s and 70s, threw light on a range of problems in the philosophy of mind. Knowing what Brentano had asked and answered before was indeed something useful to know.
3. Critical history of philosophy is itself philosophy - the two are identical
The history of philosophy can be done in at least two (if not two dozen) ways. There is the 'There was this chap, Plato, and he said' approach in which ideas and arguments are catalogued, summarised, not appraised. This is really just the history of ideas or at most exegesis. However, contrasting with this is a critical approach. Suppose you come across that favourite crux, the Cartesian Circle :
The abundant literature on the "Cartesian circle," for example, is of more than
mere historical significance. If Descartes does play off clear and distinct ideas
against God's veracity in a circular fashion, then his foundation for knowledge
is put in serious jeopardy. But to make this shift to evaluation is to move over
to philosophy. Philosophy, in its reactive phase, takes up where exegesis leaves
off. Having ascertained what another philosopher said (and meant), the
philosopher's aim as critic is to decide whether, and to what extent, it is correct
or justified. Hence philosophy, like the history of philosophy, incorporates exegesis, but unlike the history of philosophy, never counts interpretation as an end
in itself. (Mash: 292.)
To confront the Cartesian Circle as a critical historian of philosophy is to engage in a philosophical inquiry : Is Descartes guilty of vicious circularity or not ? This becomes as much a live philosophical issue for you as are the problems in the most recent philosophy textbook or journal article.