Two reflexive pronouns 'się' in one phrase
Let's say that I want to express this phrase in Polish: 'Don't be afraid to stand up.' Here are three attempts:
1) Nie bój się się podnieść. 2) Nie bój się podnieść się. 3) Nie bój się podnieść.
I have never heard that a native speaker ever uttered two się in a row, so sentence 1) sounds just, well, incorrect. Besides that, however, it doesn't seem to break any grammar rules that are known to me.
Sentence 2) should be a better alternative, however, the second się doesn't look very good at the end, as it seems to be a stylistic requirement that się never apppears at the end of a sentence, unless it's a very short 2-3 word sentence.
I could just leave out the second się, indicating that the remaining się refers to both verbs, but doesn't that make the meaning ambigious? It could also mean: Don't be afraid to lift it up. I know that podnieść is a rather transitive verb and would require an object in this case, so it doesn't really mean that what I wrote, but I think it's very likely other examples exist where this could be very problematic.
I would start with a question about what you meant by 'stand up'... didn't you mean something like "stand up for yourself"? I'd say that's "postawić się". "podnieść się" is completely literal. Maybe that's what you meant, but how often would I be afraid of that? ;)
Apart from that... that's a very interesting question. I don't know if the first two are just wrong, but true, they are strange. So the third option wins, despite the fact that the two 'się' are melted into one...
On the other hand, I can easily write a sentence with two 'się', like "Często kłócę się z nim o to, że on się nie myje". But those are different clauses...
I actually heard the first two uttered in casual speech once or twice in my life, even if they're both cringeworthy. The second one less so, especially if it has been continued.
As for why multiple "się" in one sentence sound so bad. Polish hates repetition (as Polish teachers in elementary schools do). Which means you always try to limit to the minimum the instances of the same word that are near each other, unless you absolutely can't avoid it.
One się already suggests the action in the phrase the listener directs/will direct at himself. And it has to be near "bać" which, unlike "podnosić", can't exist without "się". Although, as mihxal suggested, in this case the translation "Nie bój się wstać" would have been better anyway.
Don't be afraid to lift it up. means Nie bój się tego podnieść so it won't be ambiguous.
The second one is perfectly fine, grammatically speaking. It'd probably sound less "cringeworthy" if it wasn't sentence-final.
Maybe I am wrong, but look at "to" before "stand up". This is an infinitive and the object of the sentence. It looks like to be "detached" from the subject. "Się" refers only to "be afraid" (predicate).