What is the rule for using the "passive se" (e.g. "¿Cómo se dice?") with a reflexive verb that involves another se pronoun? For example, how would you translate "One takes a shower (ducharse) frequently in hot weather."? Do the two se pronouns simply combine into one?
As far as I know there is no "passive se".
As others have introduced, there are mainly two uses for "se".
- In reflexive verbs, like "ducharse", "se" is a particle that adds to the verb and simply means the receptor of the action: "myself", "yourself", etc. This is the same idea of any verb that uses an indirect object particle for a person (telling who receives the action).
Me da la pelota, te doy la pelota, le doy la pelota, nos da la pelota, os damos la pelota, les damos la pelota.
However, in the case of reflexive verbs, the third person changes and "se" is used in the third person (both singular and plural) instead of the corresponding "le" or "les".
Ducharse: Yo me ducho, tú te duchas, él se ducha, nosotros nos duchamos, vosotros os ducháis, ellos se duchan.
If you speak German, also a Germanic language like English, this case applies exactly in the same way:
"sich duschen": ich dusche mich, du duschest dich, er duscht sich...
- Another use of "se" is also to introduce a general/impersonal statement (but that is not passive in Spanish). It can be used with any verb and it's conjugated in the third person singular.
"Se recomienda no entrar" (entering is not recommended) "Se sabe que fue él" (it's known that it was him).
I guess your doubt is the following: can a reflexive verb (which uses the "se" particle) be combined with the impersonal "se" (what you call "the passive se") in the same sentence?
"Se recomienda ducharse primero" (taking a shower first is recommended). "Se recomienda darse crema" (using sunscreen is recommended).
Hope this helps.
I'm posting a new answer since I misunderstood and needed some more room than a comment.
Thanks for the detailed answer, although I was really wondering not whether they can be combined in a single sentence (using two verbs), but in a single impersonal/reflexive verb (like the "How does one shave?" example I gave in comments above).
In English you use an auxiliary verb "do" in "How does one shave?", but in Spanish you don't. There is only one verb in the sentence "¿Cómo se afeita uno?". We could get in an deep debate about where that "se" comes from, but I think it's safe and easy to just assume it's a fixed impersonal particle. Thinking it is a verb does not help and confuses more.
The first "se" is therefore just the "impersonal se".
Think about how you ask impersonal "how to" questions with "se" in Spanish:
¿Cómo se... conduce un coche?
¿Cómo se... estudia un idioma?
That's just a fixed formula.
A small break here that hope clarifies the thing about "se" in reflexive verbs, which is not so special after all.
In a general sentence we have a subject + verb (action) + direct object (what) + indirect object (receiver).
Yo + doy + una manzana + a María
I + give + an Apple + to Mary
As you know, you can substitute both direct and indirect object for pronouns.
I + give + it + to her
Which in Spanish can be tricky because not only is the order of the sentence altered but there is also an exception. This is the case of the indirect object "le" (to him / to her) that changes to "se" when both a direct and indirect object pronouns are present in the same sentence.
If you only use a pronoun for the indirect object, you say:
Yo + doy + la manzana + a ella
Yo + le + doy + la manzana
But if you use both pronouns, instead of the logical:
Yo + le + la + doy
Which is incorrect, "le" is changed to avoid the le-la "cacophony" (bad sound):
Yo + se + la + doy
This is where the infamous third person "se" for the indirect object comes from.
Reflexive verbs are not so special after all. It just happens that the receiver of the action (indirect object) is the same as the performer (subject), so logically speaking they are simply a normal verb with an indirect object pronoun sticked to them to mark that they can be applied to oneself.
"Ducharse" is just a special case of "duchar" (you can "duchar a alguien" / shower somebody). Ducharse just means that this verb can be applied to oneself, and the one who's being showered is the same as "the showerer" :):
Yo ducho a mí
Tu duchas a ti
Él ducha a él. By the way... who is "él"? "Él" himself, or "él" another other guy???
But that sounds weird so we just use reflexive pronouns (which as you see have a lot to do with indirect object personal pronouns), that in the third person is "se" to clarify that te showering is to oneself, not to a third person.
Yo me ducho
Tu te duchas
El se ducha
And now, back to the original answer.
Remember the general impersonal form of asking "how to"?
¿Cómo se... conduce un coche?
Why should reflexive verbs be special? We just say "uno" (meaning "un hombre", "uno a sí mismo") just like in English you say "oneself".
¿Cómo se... afeita uno?
If it's referred specifically to a girl, then "una" is generally used:
¿Cómo se... maquilla una?
Pfew. Hope it's clear now.
I just wanted to make clear where the "reflexive se" comes from and what the relation is.